[v5,1/8] xen/arm: introduce static shared memory

Commit Message

Penny Zheng June 20, 2022, 5:11 a.m. UTC

From: Penny Zheng <[e-mail]>

This patch serie introduces a new feature: setting up static
shared memory on a dom0less system, through device tree configuration.

This commit parses shared memory node at boot-time, and reserve it in
bootinfo.reserved_mem to avoid other use.

This commits proposes a new Kconfig CONFIG_STATIC_SHM to wrap
static-shm-related codes, and this option depends on static memory(
CONFIG_STATIC_MEMORY). That's because that later we want to reuse a few
helpers, guarded with CONFIG_STATIC_MEMORY, like acquire_staticmem_pages, etc,
on static shared memory.

Signed-off-by: Penny Zheng <[e-mail]>
Reviewed-by: Stefano Stabellini <[e-mail]>
---
v5 change:
- no change
---
v4 change:
- nit fix on doc
---
v3 change:
- make nr_shm_domain unsigned int
---
v2 change:
- document refinement
- remove bitmap and use the iteration to check
- add a new field nr_shm_domain to keep the number of shared domain
---
 docs/misc/arm/device-tree/booting.txt | 120 ++++++++++++++++++++++++++
 xen/arch/arm/Kconfig                  |   6 ++
 xen/arch/arm/bootfdt.c                |  68 +++++++++++++++
 xen/arch/arm/include/asm/setup.h      |   3 +
 4 files changed, 197 insertions(+)

Comments

Julien Grall June 24, 2022, 5:55 p.m. UTC | #1

Hi Penny,

On 20/06/2022 06:11, Penny Zheng wrote:
> From: Penny Zheng <[e-mail]>
> 
> This patch serie introduces a new feature: setting up static

Typo: s/serie/series/

> shared memory on a dom0less system, through device tree configuration.
> 
> This commit parses shared memory node at boot-time, and reserve it in
> bootinfo.reserved_mem to avoid other use.
> 
> This commits proposes a new Kconfig CONFIG_STATIC_SHM to wrap
> static-shm-related codes, and this option depends on static memory(
> CONFIG_STATIC_MEMORY). That's because that later we want to reuse a few
> helpers, guarded with CONFIG_STATIC_MEMORY, like acquire_staticmem_pages, etc,
> on static shared memory.
> 
> Signed-off-by: Penny Zheng <[e-mail]>
> Reviewed-by: Stefano Stabellini <[e-mail]>
> ---
> v5 change:
> - no change
> ---
> v4 change:
> - nit fix on doc
> ---
> v3 change:
> - make nr_shm_domain unsigned int
> ---
> v2 change:
> - document refinement
> - remove bitmap and use the iteration to check
> - add a new field nr_shm_domain to keep the number of shared domain
> ---
>   docs/misc/arm/device-tree/booting.txt | 120 ++++++++++++++++++++++++++
>   xen/arch/arm/Kconfig                  |   6 ++
>   xen/arch/arm/bootfdt.c                |  68 +++++++++++++++
>   xen/arch/arm/include/asm/setup.h      |   3 +
>   4 files changed, 197 insertions(+)
> 
> diff --git a/docs/misc/arm/device-tree/booting.txt b/docs/misc/arm/device-tree/booting.txt
> index 98253414b8..6467bc5a28 100644
> --- a/docs/misc/arm/device-tree/booting.txt
> +++ b/docs/misc/arm/device-tree/booting.txt
> @@ -378,3 +378,123 @@ device-tree:
>   
>   This will reserve a 512MB region starting at the host physical address
>   0x30000000 to be exclusively used by DomU1.
> +
> +Static Shared Memory
> +====================
> +
> +The static shared memory device tree nodes allow users to statically set up
> +shared memory on dom0less system, enabling domains to do shm-based
> +communication.
> +
> +- compatible
> +
> +    "xen,domain-shared-memory-v1"
> +
> +- xen,shm-id
> +
> +    An 8-bit integer that represents the unique identifier of the shared memory
> +    region. The maximum identifier shall be "xen,shm-id = <0xff>".
> +
> +- xen,shared-mem
> +
> +    An array takes a physical address, which is the base address of the
> +    shared memory region in host physical address space, a size, and a guest
> +    physical address, as the target address of the mapping. The number of cells
> +    for the host address (and size) is the same as the guest pseudo-physical
> +    address and they are inherited from the parent node.

Sorry for jump in the discussion late. But as this is going to be a 
stable ABI, I would to make sure the interface is going to be easily 
extendable.

AFAIU, with your proposal the host physical address is mandatory. I 
would expect that some user may want to share memory but don't care 
about the exact location in memory. So I think it would be good to make 
it optional in the binding.

I think this wants to be done now because it would be difficult to 
change the binding afterwards (the host physical address is the first 
set of cells).

The Xen doesn't need to handle the optional case.

[...]

> diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
> index be9eff0141..7321f47c0f 100644
> --- a/xen/arch/arm/Kconfig
> +++ b/xen/arch/arm/Kconfig
> @@ -139,6 +139,12 @@ config TEE
>   
>   source "arch/arm/tee/Kconfig"
>   
> +config STATIC_SHM
> +	bool "Statically shared memory on a dom0less system" if UNSUPPORTED

You also want to update SUPPORT.md.

> +	depends on STATIC_MEMORY
> +	help
> +	  This option enables statically shared memory on a dom0less system.
> +
>   endmenu
>   
>   menu "ARM errata workaround via the alternative framework"
> diff --git a/xen/arch/arm/bootfdt.c b/xen/arch/arm/bootfdt.c
> index ec81a45de9..38dcb05d5d 100644
> --- a/xen/arch/arm/bootfdt.c
> +++ b/xen/arch/arm/bootfdt.c
> @@ -361,6 +361,70 @@ static int __init process_domain_node(const void *fdt, int node,
>                                      size_cells, &bootinfo.reserved_mem, true);
>   }
>   
> +#ifdef CONFIG_STATIC_SHM
> +static int __init process_shm_node(const void *fdt, int node,
> +                                   u32 address_cells, u32 size_cells)
> +{
> +    const struct fdt_property *prop;
> +    const __be32 *cell;
> +    paddr_t paddr, size;
> +    struct meminfo *mem = &bootinfo.reserved_mem;
> +    unsigned long i;

nr_banks is "unsigned int" so I think this should be "unsigned int" as well.

> +
> +    if ( address_cells < 1 || size_cells < 1 )
> +    {
> +        printk("fdt: invalid #address-cells or #size-cells for static shared memory node.\n");
> +        return -EINVAL;
> +    }
> +
> +    prop = fdt_get_property(fdt, node, "xen,shared-mem", NULL);
> +    if ( !prop )
> +        return -ENOENT;
> +
> +    /*
> +     * xen,shared-mem = <paddr, size, gaddr>;
> +     * Memory region starting from physical address #paddr of #size shall
> +     * be mapped to guest physical address #gaddr as static shared memory
> +     * region.
> +     */
> +    cell = (const __be32 *)prop->data;
> +    device_tree_get_reg(&cell, address_cells, size_cells, &paddr, &size);

Please check the len of the property to confirm is it big enough to 
contain "paddr", "size", and "gaddr".

> +    for ( i = 0; i < mem->nr_banks; i++ )
> +    {
> +        /*
> +         * A static shared memory region could be shared between multiple
> +         * domains.
> +         */
> +        if ( paddr == mem->bank[i].start && size == mem->bank[i].size )
> +            break;
> +    }
> +
> +    if ( i == mem->nr_banks )
> +    {
> +        if ( i < NR_MEM_BANKS )
> +        {
> +            /* Static shared memory shall be reserved from any other use. */
> +            mem->bank[mem->nr_banks].start = paddr;
> +            mem->bank[mem->nr_banks].size = size;
> +            mem->bank[mem->nr_banks].xen_domain = true;
> +            mem->nr_banks++;
> +        }
> +        else
> +        {
> +            printk("Warning: Max number of supported memory regions reached.\n");
> +            return -ENOSPC;
> +        }
> +    }
> +    /*
> +     * keep a count of the number of domains, which later may be used to
> +     * calculate the number of the reference count.
> +     */
> +    mem->bank[i].nr_shm_domain++;
> +
> +    return 0;
> +}
> +#endif
> +
>   static int __init early_scan_node(const void *fdt,
>                                     int node, const char *name, int depth,
>                                     u32 address_cells, u32 size_cells,
> @@ -386,6 +450,10 @@ static int __init early_scan_node(const void *fdt,
>           process_chosen_node(fdt, node, name, address_cells, size_cells);
>       else if ( depth == 2 && device_tree_node_compatible(fdt, node, "xen,domain") )
>           rc = process_domain_node(fdt, node, name, address_cells, size_cells);
> +#ifdef CONFIG_STATIC_SHM
> +    else if ( depth <= 3 && device_tree_node_compatible(fdt, node, "xen,domain-shared-memory-v1") )
> +        rc = process_shm_node(fdt, node, address_cells, size_cells);
> +#endif
>   
>       if ( rc < 0 )
>           printk("fdt: node `%s': parsing failed\n", name);
> diff --git a/xen/arch/arm/include/asm/setup.h b/xen/arch/arm/include/asm/setup.h
> index 2bb01ecfa8..5063e5d077 100644
> --- a/xen/arch/arm/include/asm/setup.h
> +++ b/xen/arch/arm/include/asm/setup.h
> @@ -27,6 +27,9 @@ struct membank {
>       paddr_t start;
>       paddr_t size;
>       bool xen_domain; /* whether the memory bank is bound to a Xen domain. */
> +#ifdef CONFIG_STATIC_SHM
> +    unsigned int nr_shm_domain;
> +#endif
>   };
>   
>   struct meminfo {

Cheers,

Julien Grall June 24, 2022, 7:25 p.m. UTC | #2

Hi Penny,

I have looked at the code and I have further questions about the binding.

On 20/06/2022 06:11, Penny Zheng wrote:
> ---
>   docs/misc/arm/device-tree/booting.txt | 120 ++++++++++++++++++++++++++
>   xen/arch/arm/Kconfig                  |   6 ++
>   xen/arch/arm/bootfdt.c                |  68 +++++++++++++++
>   xen/arch/arm/include/asm/setup.h      |   3 +
>   4 files changed, 197 insertions(+)
> 
> diff --git a/docs/misc/arm/device-tree/booting.txt b/docs/misc/arm/device-tree/booting.txt
> index 98253414b8..6467bc5a28 100644
> --- a/docs/misc/arm/device-tree/booting.txt
> +++ b/docs/misc/arm/device-tree/booting.txt
> @@ -378,3 +378,123 @@ device-tree:
>   
>   This will reserve a 512MB region starting at the host physical address
>   0x30000000 to be exclusively used by DomU1.
> +
> +Static Shared Memory
> +====================
> +
> +The static shared memory device tree nodes allow users to statically set up
> +shared memory on dom0less system, enabling domains to do shm-based
> +communication.
> +
> +- compatible
> +
> +    "xen,domain-shared-memory-v1"
> +
> +- xen,shm-id
> +
> +    An 8-bit integer that represents the unique identifier of the shared memory
> +    region. The maximum identifier shall be "xen,shm-id = <0xff>".

There is nothing in Xen that will ensure that xen,shm-id will match for 
all the nodes using the same region.

I see you write it to the guest device-tree. However there is a mismatch 
of the type: here you use an integer whereas the guest binding is using 
a string.

Cheers,

Penny Zheng June 29, 2022, 5:38 a.m. UTC | #3

Hi Julien

> -----Original Message-----
> From: Julien Grall <[e-mail]>
> Sent: Saturday, June 25, 2022 1:55 AM
> To: Penny Zheng <[e-mail]>; [e-mail]
> Cc: Wei Chen <[e-mail]>; Stefano Stabellini
> <[e-mail]>; Bertrand Marquis <[e-mail]>;
> Volodymyr Babchuk <[e-mail]>
> Subject: Re: [PATCH v5 1/8] xen/arm: introduce static shared memory
> 
> Hi Penny,
> 
> On 20/06/2022 06:11, Penny Zheng wrote:
> > From: Penny Zheng <[e-mail]>
> >
> > This patch serie introduces a new feature: setting up static
> 
> Typo: s/serie/series/
> 
> > shared memory on a dom0less system, through device tree configuration.
> >
> > This commit parses shared memory node at boot-time, and reserve it in
> > bootinfo.reserved_mem to avoid other use.
> >
> > This commits proposes a new Kconfig CONFIG_STATIC_SHM to wrap
> > static-shm-related codes, and this option depends on static memory(
> > CONFIG_STATIC_MEMORY). That's because that later we want to reuse a
> > few helpers, guarded with CONFIG_STATIC_MEMORY, like
> > acquire_staticmem_pages, etc, on static shared memory.
> >
> > Signed-off-by: Penny Zheng <[e-mail]>
> > Reviewed-by: Stefano Stabellini <[e-mail]>
> > ---
> > v5 change:
> > - no change
> > ---
> > v4 change:
> > - nit fix on doc
> > ---
> > v3 change:
> > - make nr_shm_domain unsigned int
> > ---
> > v2 change:
> > - document refinement
> > - remove bitmap and use the iteration to check
> > - add a new field nr_shm_domain to keep the number of shared domain
> > ---
> >   docs/misc/arm/device-tree/booting.txt | 120
> ++++++++++++++++++++++++++
> >   xen/arch/arm/Kconfig                  |   6 ++
> >   xen/arch/arm/bootfdt.c                |  68 +++++++++++++++
> >   xen/arch/arm/include/asm/setup.h      |   3 +
> >   4 files changed, 197 insertions(+)
> >
> > diff --git a/docs/misc/arm/device-tree/booting.txt
> > b/docs/misc/arm/device-tree/booting.txt
> > index 98253414b8..6467bc5a28 100644
> > --- a/docs/misc/arm/device-tree/booting.txt
> > +++ b/docs/misc/arm/device-tree/booting.txt
> > @@ -378,3 +378,123 @@ device-tree:
> >
> >   This will reserve a 512MB region starting at the host physical address
> >   0x30000000 to be exclusively used by DomU1.
> > +
> > +Static Shared Memory
> > +====================
> > +
> > +The static shared memory device tree nodes allow users to statically
> > +set up shared memory on dom0less system, enabling domains to do
> > +shm-based communication.
> > +
> > +- compatible
> > +
> > +    "xen,domain-shared-memory-v1"
> > +
> > +- xen,shm-id
> > +
> > +    An 8-bit integer that represents the unique identifier of the shared
> memory
> > +    region. The maximum identifier shall be "xen,shm-id = <0xff>".
> > +
> > +- xen,shared-mem
> > +
> > +    An array takes a physical address, which is the base address of the
> > +    shared memory region in host physical address space, a size, and a
> guest
> > +    physical address, as the target address of the mapping. The number of
> cells
> > +    for the host address (and size) is the same as the guest pseudo-physical
> > +    address and they are inherited from the parent node.
> 
> Sorry for jump in the discussion late. But as this is going to be a stable ABI, I
> would to make sure the interface is going to be easily extendable.
> 
> AFAIU, with your proposal the host physical address is mandatory. I would
> expect that some user may want to share memory but don't care about the
> exact location in memory. So I think it would be good to make it optional in
> the binding.
> 
> I think this wants to be done now because it would be difficult to change the
> binding afterwards (the host physical address is the first set of cells).
> 
> The Xen doesn't need to handle the optional case.
> 

Sure, I'll make "the host physical address" optional here, and right now, with no actual
code implementation. I'll make up it later in free time~

The user case you mentioned here is that we let xen to allocate an arbitrary static shared
memory region, so size and guest physical address are still mandatory, right?
 
> [...]
> 
> > diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig index
> > be9eff0141..7321f47c0f 100644
> > --- a/xen/arch/arm/Kconfig
> > +++ b/xen/arch/arm/Kconfig
> > @@ -139,6 +139,12 @@ config TEE
> >
> >   source "arch/arm/tee/Kconfig"
> >
> > +config STATIC_SHM
> > +	bool "Statically shared memory on a dom0less system" if
> UNSUPPORTED
> 
> You also want to update SUPPORT.md.
>

Right.

> > +	depends on STATIC_MEMORY
> > +	help
> > +	  This option enables statically shared memory on a dom0less system.
> > +
> >   endmenu
> >
> >   menu "ARM errata workaround via the alternative framework"
> > diff --git a/xen/arch/arm/bootfdt.c b/xen/arch/arm/bootfdt.c index
> > ec81a45de9..38dcb05d5d 100644
> > --- a/xen/arch/arm/bootfdt.c
> > +++ b/xen/arch/arm/bootfdt.c
> > @@ -361,6 +361,70 @@ static int __init process_domain_node(const void
> *fdt, int node,
> >                                      size_cells, &bootinfo.reserved_mem, true);
> >   }
> >
> > +#ifdef CONFIG_STATIC_SHM
> > +static int __init process_shm_node(const void *fdt, int node,
> > +                                   u32 address_cells, u32 size_cells)
> > +{
> > +    const struct fdt_property *prop;
> > +    const __be32 *cell;
> > +    paddr_t paddr, size;
> > +    struct meminfo *mem = &bootinfo.reserved_mem;
> > +    unsigned long i;
> 
> nr_banks is "unsigned int" so I think this should be "unsigned int" as well.
> 

Right

> > +
> > +    if ( address_cells < 1 || size_cells < 1 )
> > +    {
> > +        printk("fdt: invalid #address-cells or #size-cells for static shared
> memory node.\n");
> > +        return -EINVAL;
> > +    }
> > +
> > +    prop = fdt_get_property(fdt, node, "xen,shared-mem", NULL);
> > +    if ( !prop )
> > +        return -ENOENT;
> > +
> > +    /*
> > +     * xen,shared-mem = <paddr, size, gaddr>;
> > +     * Memory region starting from physical address #paddr of #size shall
> > +     * be mapped to guest physical address #gaddr as static shared memory
> > +     * region.
> > +     */
> > +    cell = (const __be32 *)prop->data;
> > +    device_tree_get_reg(&cell, address_cells, size_cells, &paddr,
> > + &size);
> 
> Please check the len of the property to confirm is it big enough to contain
> "paddr", "size", and "gaddr".
> 

Sure, will do

> > +    for ( i = 0; i < mem->nr_banks; i++ )
> > +    {
> > +        /*
> > +         * A static shared memory region could be shared between multiple
> > +         * domains.
> > +         */
> > +        if ( paddr == mem->bank[i].start && size == mem->bank[i].size )
> > +            break;
> > +    }
> > +
> > +    if ( i == mem->nr_banks )
> > +    {
> > +        if ( i < NR_MEM_BANKS )
> > +        {
> > +            /* Static shared memory shall be reserved from any other use. */
> > +            mem->bank[mem->nr_banks].start = paddr;
> > +            mem->bank[mem->nr_banks].size = size;
> > +            mem->bank[mem->nr_banks].xen_domain = true;
> > +            mem->nr_banks++;
> > +        }
> > +        else
> > +        {
> > +            printk("Warning: Max number of supported memory regions
> reached.\n");
> > +            return -ENOSPC;
> > +        }
> > +    }
> > +    /*
> > +     * keep a count of the number of domains, which later may be used to
> > +     * calculate the number of the reference count.
> > +     */
> > +    mem->bank[i].nr_shm_domain++;
> > +
> > +    return 0;
> > +}
> > +#endif
> > +
> >   static int __init early_scan_node(const void *fdt,
> >                                     int node, const char *name, int depth,
> >                                     u32 address_cells, u32 size_cells,
> > @@ -386,6 +450,10 @@ static int __init early_scan_node(const void *fdt,
> >           process_chosen_node(fdt, node, name, address_cells, size_cells);
> >       else if ( depth == 2 && device_tree_node_compatible(fdt, node,
> "xen,domain") )
> >           rc = process_domain_node(fdt, node, name, address_cells,
> > size_cells);
> > +#ifdef CONFIG_STATIC_SHM
> > +    else if ( depth <= 3 && device_tree_node_compatible(fdt, node,
> "xen,domain-shared-memory-v1") )
> > +        rc = process_shm_node(fdt, node, address_cells, size_cells);
> > +#endif
> >
> >       if ( rc < 0 )
> >           printk("fdt: node `%s': parsing failed\n", name); diff --git
> > a/xen/arch/arm/include/asm/setup.h
> b/xen/arch/arm/include/asm/setup.h
> > index 2bb01ecfa8..5063e5d077 100644
> > --- a/xen/arch/arm/include/asm/setup.h
> > +++ b/xen/arch/arm/include/asm/setup.h
> > @@ -27,6 +27,9 @@ struct membank {
> >       paddr_t start;
> >       paddr_t size;
> >       bool xen_domain; /* whether the memory bank is bound to a Xen
> > domain. */
> > +#ifdef CONFIG_STATIC_SHM
> > +    unsigned int nr_shm_domain;
> > +#endif
> >   };
> >
> >   struct meminfo {
> 
> Cheers,
> 
> --
> Julien Grall

Penny Zheng June 29, 2022, 8:39 a.m. UTC | #4

Hi Julien

> -----Original Message-----
> From: Julien Grall <[e-mail]>
> Sent: Saturday, June 25, 2022 1:55 AM
> To: Penny Zheng <[e-mail]>; [e-mail]
> Cc: Wei Chen <[e-mail]>; Stefano Stabellini
> <[e-mail]>; Bertrand Marquis <[e-mail]>;
> Volodymyr Babchuk <[e-mail]>
> Subject: Re: [PATCH v5 1/8] xen/arm: introduce static shared memory
> 
> Hi Penny,
> 
> On 20/06/2022 06:11, Penny Zheng wrote:
> > From: Penny Zheng <[e-mail]>
> >
> > This patch serie introduces a new feature: setting up static
> 
> Typo: s/serie/series/
> 
> > shared memory on a dom0less system, through device tree configuration.
> >
> > This commit parses shared memory node at boot-time, and reserve it in
> > bootinfo.reserved_mem to avoid other use.
> >
> > This commits proposes a new Kconfig CONFIG_STATIC_SHM to wrap
> > static-shm-related codes, and this option depends on static memory(
> > CONFIG_STATIC_MEMORY). That's because that later we want to reuse a
> > few helpers, guarded with CONFIG_STATIC_MEMORY, like
> > acquire_staticmem_pages, etc, on static shared memory.
> >
> > Signed-off-by: Penny Zheng <[e-mail]>
> > Reviewed-by: Stefano Stabellini <[e-mail]>
> > ---
> > v5 change:
> > - no change
> > ---
> > v4 change:
> > - nit fix on doc
> > ---
> > v3 change:
> > - make nr_shm_domain unsigned int
> > ---
> > v2 change:
> > - document refinement
> > - remove bitmap and use the iteration to check
> > - add a new field nr_shm_domain to keep the number of shared domain
> > ---
> >   docs/misc/arm/device-tree/booting.txt | 120
> ++++++++++++++++++++++++++
> >   xen/arch/arm/Kconfig                  |   6 ++
> >   xen/arch/arm/bootfdt.c                |  68 +++++++++++++++
> >   xen/arch/arm/include/asm/setup.h      |   3 +
> >   4 files changed, 197 insertions(+)
> >
> > diff --git a/docs/misc/arm/device-tree/booting.txt
> > b/docs/misc/arm/device-tree/booting.txt
> > index 98253414b8..6467bc5a28 100644
> > --- a/docs/misc/arm/device-tree/booting.txt
> > +++ b/docs/misc/arm/device-tree/booting.txt
> > @@ -378,3 +378,123 @@ device-tree:
> >
> >   This will reserve a 512MB region starting at the host physical address
> >   0x30000000 to be exclusively used by DomU1.
> > +
> > +Static Shared Memory
> > +====================
> > +
> > +The static shared memory device tree nodes allow users to statically
> > +set up shared memory on dom0less system, enabling domains to do
> > +shm-based communication.
> > +
> > +- compatible
> > +
> > +    "xen,domain-shared-memory-v1"
> > +
> > +- xen,shm-id
> > +
> > +    An 8-bit integer that represents the unique identifier of the shared
> memory
> > +    region. The maximum identifier shall be "xen,shm-id = <0xff>".
> > +
> > +- xen,shared-mem
> > +
> > +    An array takes a physical address, which is the base address of the
> > +    shared memory region in host physical address space, a size, and a
> guest
> > +    physical address, as the target address of the mapping. The number of
> cells
> > +    for the host address (and size) is the same as the guest pseudo-physical
> > +    address and they are inherited from the parent node.
> 
> Sorry for jump in the discussion late. But as this is going to be a stable ABI, I
> would to make sure the interface is going to be easily extendable.
> 
> AFAIU, with your proposal the host physical address is mandatory. I would
> expect that some user may want to share memory but don't care about the
> exact location in memory. So I think it would be good to make it optional in
> the binding.
> 
> I think this wants to be done now because it would be difficult to change the
> binding afterwards (the host physical address is the first set of cells).
> 
> The Xen doesn't need to handle the optional case.
> 
> [...]
> 
> > diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig index
> > be9eff0141..7321f47c0f 100644
> > --- a/xen/arch/arm/Kconfig
> > +++ b/xen/arch/arm/Kconfig
> > @@ -139,6 +139,12 @@ config TEE
> >
> >   source "arch/arm/tee/Kconfig"
> >
> > +config STATIC_SHM
> > +	bool "Statically shared memory on a dom0less system" if
> UNSUPPORTED
> 
> You also want to update SUPPORT.md.
> 
> > +	depends on STATIC_MEMORY
> > +	help
> > +	  This option enables statically shared memory on a dom0less system.
> > +
> >   endmenu
> >
> >   menu "ARM errata workaround via the alternative framework"
> > diff --git a/xen/arch/arm/bootfdt.c b/xen/arch/arm/bootfdt.c index
> > ec81a45de9..38dcb05d5d 100644
> > --- a/xen/arch/arm/bootfdt.c
> > +++ b/xen/arch/arm/bootfdt.c
> > @@ -361,6 +361,70 @@ static int __init process_domain_node(const void
> *fdt, int node,
> >                                      size_cells, &bootinfo.reserved_mem, true);
> >   }
> >
> > +#ifdef CONFIG_STATIC_SHM
> > +static int __init process_shm_node(const void *fdt, int node,
> > +                                   u32 address_cells, u32 size_cells)
> > +{
> > +    const struct fdt_property *prop;
> > +    const __be32 *cell;
> > +    paddr_t paddr, size;
> > +    struct meminfo *mem = &bootinfo.reserved_mem;
> > +    unsigned long i;
> 
> nr_banks is "unsigned int" so I think this should be "unsigned int" as well.
> 
> > +
> > +    if ( address_cells < 1 || size_cells < 1 )
> > +    {
> > +        printk("fdt: invalid #address-cells or #size-cells for static shared
> memory node.\n");
> > +        return -EINVAL;
> > +    }
> > +
> > +    prop = fdt_get_property(fdt, node, "xen,shared-mem", NULL);
> > +    if ( !prop )
> > +        return -ENOENT;
> > +
> > +    /*
> > +     * xen,shared-mem = <paddr, size, gaddr>;
> > +     * Memory region starting from physical address #paddr of #size shall
> > +     * be mapped to guest physical address #gaddr as static shared memory
> > +     * region.
> > +     */
> > +    cell = (const __be32 *)prop->data;
> > +    device_tree_get_reg(&cell, address_cells, size_cells, &paddr,
> > + &size);
> 
> Please check the len of the property to confirm is it big enough to contain
> "paddr", "size", and "gaddr".
> 
> > +    for ( i = 0; i < mem->nr_banks; i++ )
> > +    {
> > +        /*
> > +         * A static shared memory region could be shared between multiple
> > +         * domains.
> > +         */
> > +        if ( paddr == mem->bank[i].start && size == mem->bank[i].size )
> > +            break;

Maybe I need to add a check on shm-id:
"
        /*
         * A static shared memory region could be shared between multiple
         * domains.
         */
        if ( strcmp(shm_id, mem->bank[i].shm_id) == 0 )
        {
            if ( paddr == mem->bank[i].start && size == mem->bank[i].size )
                break;
            else
            {
                printk("Warning: xen,shm-id %s does not match for all the nodes using the same region.\n",
                       shm_id);
                return -EINVAL;
            }
        }
"
Wdyt？

> > +    }
> > +
> > +    if ( i == mem->nr_banks )
> > +    {
> > +        if ( i < NRshm_MEM_BANKS )
> > +        {
> > +            /* Static shared memory shall be reserved from any other use. */
> > +            mem->bank[mem->nr_banks].start = paddr;
> > +            mem->bank[mem->nr_banks].size = size;
> > +            mem->bank[mem->nr_banks].xen_domain = true;
> > +            mem->nr_banks++;
> > +        }
> > +        else
> > +        {
> > +            printk("Warning: Max number of supported memory regions
> reached.\n");
> > +            return -ENOSPC;
> > +        }
> > +    }
> > +    /*
> > +     * keep a count of the number of domains, which later may be used to
> > +     * calculate the number of the reference count.
> > +     */
> > +    mem->bank[i].nr_shm_domain++;
> > +
> > +    return 0;
> > +}
> > +#endif
> > +
> >   static int __init early_scan_node(const void *fdt,
> >                                     int node, const char *name, int depth,
> >                                     u32 address_cells, u32 size_cells,
> > @@ -386,6 +450,10 @@ static int __init early_scan_node(const void *fdt,
> >           process_chosen_node(fdt, node, name, address_cells, size_cells);
> >       else if ( depth == 2 && device_tree_node_compatible(fdt, node,
> "xen,domain") )
> >           rc = process_domain_node(fdt, node, name, address_cells,
> > size_cells);
> > +#ifdef CONFIG_STATIC_SHM
> > +    else if ( depth <= 3 && device_tree_node_compatible(fdt, node,
> "xen,domain-shared-memory-v1") )
> > +        rc = process_shm_node(fdt, node, address_cells, size_cells);
> > +#endif
> >
> >       if ( rc < 0 )
> >           printk("fdt: node `%s': parsing failed\n", name); diff --git
> > a/xen/arch/arm/include/asm/setup.h
> b/xen/arch/arm/include/asm/setup.h
> > index 2bb01ecfa8..5063e5d077 100644
> > --- a/xen/arch/arm/include/asm/setup.h
> > +++ b/xen/arch/arm/include/asm/setup.h
> > @@ -27,6 +27,9 @@ struct membank {
> >       paddr_t start;
> >       paddr_t size;
> >       bool xen_domain; /* whether the memory bank is bound to a Xen
> > domain. */
> > +#ifdef CONFIG_STATIC_SHM
> > +    unsigned int nr_shm_domain;
> > +#endif
> >   };
> >
> >   struct meminfo {
> 
> Cheers,
> 
> --
> Julien Grall

Penny Zheng June 29, 2022, 8:40 a.m. UTC | #5

Hi Julien

> -----Original Message-----
> From: Julien Grall <[e-mail]>
> Sent: Saturday, June 25, 2022 3:26 AM
> To: Penny Zheng <[e-mail]>; [e-mail]
> Cc: Wei Chen <[e-mail]>; Stefano Stabellini
> <[e-mail]>; Bertrand Marquis <[e-mail]>;
> Volodymyr Babchuk <[e-mail]>
> Subject: Re: [PATCH v5 1/8] xen/arm: introduce static shared memory
> 
> Hi Penny,
> 
> I have looked at the code and I have further questions about the binding.
> 
> On 20/06/2022 06:11, Penny Zheng wrote:
> > ---
> >   docs/misc/arm/device-tree/booting.txt | 120
> ++++++++++++++++++++++++++
> >   xen/arch/arm/Kconfig                  |   6 ++
> >   xen/arch/arm/bootfdt.c                |  68 +++++++++++++++
> >   xen/arch/arm/include/asm/setup.h      |   3 +
> >   4 files changed, 197 insertions(+)
> >
> > diff --git a/docs/misc/arm/device-tree/booting.txt
> > b/docs/misc/arm/device-tree/booting.txt
> > index 98253414b8..6467bc5a28 100644
> > --- a/docs/misc/arm/device-tree/booting.txt
> > +++ b/docs/misc/arm/device-tree/booting.txt
> > @@ -378,3 +378,123 @@ device-tree:
> >
> >   This will reserve a 512MB region starting at the host physical address
> >   0x30000000 to be exclusively used by DomU1.
> > +
> > +Static Shared Memory
> > +====================
> > +
> > +The static shared memory device tree nodes allow users to statically
> > +set up shared memory on dom0less system, enabling domains to do
> > +shm-based communication.
> > +
> > +- compatible
> > +
> > +    "xen,domain-shared-memory-v1"
> > +
> > +- xen,shm-id
> > +
> > +    An 8-bit integer that represents the unique identifier of the shared
> memory
> > +    region. The maximum identifier shall be "xen,shm-id = <0xff>".
> 
> There is nothing in Xen that will ensure that xen,shm-id will match for all the
> nodes using the same region.
> 

True, we actually do not use this field, adding it here to just be aligned with Linux.
I could add a check in the very beginning when we parse the device tree.
I'll give more details to explain in which code locates.

> I see you write it to the guest device-tree. However there is a mismatch of the
> type: here you use an integer whereas the guest binding is using a string.
> 
> Cheers,
> 
> --
> Julien Grall

Julien Grall June 29, 2022, 10:17 a.m. UTC | #6

On 29/06/2022 06:38, Penny Zheng wrote:
> Hi Julien

Hi Penny,

> 
>> -----Original Message-----
>> From: Julien Grall <[e-mail]>
>> Sent: Saturday, June 25, 2022 1:55 AM
>> To: Penny Zheng <[e-mail]>; [e-mail]
>> Cc: Wei Chen <[e-mail]>; Stefano Stabellini
>> <[e-mail]>; Bertrand Marquis <[e-mail]>;
>> Volodymyr Babchuk <[e-mail]>
>> Subject: Re: [PATCH v5 1/8] xen/arm: introduce static shared memory
>>
>> Hi Penny,
>>
>> On 20/06/2022 06:11, Penny Zheng wrote:
>>> From: Penny Zheng <[e-mail]>
>>>
>>> This patch serie introduces a new feature: setting up static
>>
>> Typo: s/serie/series/
>>
>>> shared memory on a dom0less system, through device tree configuration.
>>>
>>> This commit parses shared memory node at boot-time, and reserve it in
>>> bootinfo.reserved_mem to avoid other use.
>>>
>>> This commits proposes a new Kconfig CONFIG_STATIC_SHM to wrap
>>> static-shm-related codes, and this option depends on static memory(
>>> CONFIG_STATIC_MEMORY). That's because that later we want to reuse a
>>> few helpers, guarded with CONFIG_STATIC_MEMORY, like
>>> acquire_staticmem_pages, etc, on static shared memory.
>>>
>>> Signed-off-by: Penny Zheng <[e-mail]>
>>> Reviewed-by: Stefano Stabellini <[e-mail]>
>>> ---
>>> v5 change:
>>> - no change
>>> ---
>>> v4 change:
>>> - nit fix on doc
>>> ---
>>> v3 change:
>>> - make nr_shm_domain unsigned int
>>> ---
>>> v2 change:
>>> - document refinement
>>> - remove bitmap and use the iteration to check
>>> - add a new field nr_shm_domain to keep the number of shared domain
>>> ---
>>>    docs/misc/arm/device-tree/booting.txt | 120
>> ++++++++++++++++++++++++++
>>>    xen/arch/arm/Kconfig                  |   6 ++
>>>    xen/arch/arm/bootfdt.c                |  68 +++++++++++++++
>>>    xen/arch/arm/include/asm/setup.h      |   3 +
>>>    4 files changed, 197 insertions(+)
>>>
>>> diff --git a/docs/misc/arm/device-tree/booting.txt
>>> b/docs/misc/arm/device-tree/booting.txt
>>> index 98253414b8..6467bc5a28 100644
>>> --- a/docs/misc/arm/device-tree/booting.txt
>>> +++ b/docs/misc/arm/device-tree/booting.txt
>>> @@ -378,3 +378,123 @@ device-tree:
>>>
>>>    This will reserve a 512MB region starting at the host physical address
>>>    0x30000000 to be exclusively used by DomU1.
>>> +
>>> +Static Shared Memory
>>> +====================
>>> +
>>> +The static shared memory device tree nodes allow users to statically
>>> +set up shared memory on dom0less system, enabling domains to do
>>> +shm-based communication.
>>> +
>>> +- compatible
>>> +
>>> +    "xen,domain-shared-memory-v1"
>>> +
>>> +- xen,shm-id
>>> +
>>> +    An 8-bit integer that represents the unique identifier of the shared
>> memory
>>> +    region. The maximum identifier shall be "xen,shm-id = <0xff>".
>>> +
>>> +- xen,shared-mem
>>> +
>>> +    An array takes a physical address, which is the base address of the
>>> +    shared memory region in host physical address space, a size, and a
>> guest
>>> +    physical address, as the target address of the mapping. The number of
>> cells
>>> +    for the host address (and size) is the same as the guest pseudo-physical
>>> +    address and they are inherited from the parent node.
>>
>> Sorry for jump in the discussion late. But as this is going to be a stable ABI, I
>> would to make sure the interface is going to be easily extendable.
>>
>> AFAIU, with your proposal the host physical address is mandatory. I would
>> expect that some user may want to share memory but don't care about the
>> exact location in memory. So I think it would be good to make it optional in
>> the binding.
>>
>> I think this wants to be done now because it would be difficult to change the
>> binding afterwards (the host physical address is the first set of cells).
>>
>> The Xen doesn't need to handle the optional case.
>>
> 
> Sure, I'll make "the host physical address" optional here, and right now, with no actual
> code implementation. I'll make up it later in free time~
> 
> The user case you mentioned here is that we let xen to allocate an arbitrary static shared
> memory region, so size and guest physical address are still mandatory, right?

That's correct.

Cheers,

Penny Zheng July 13, 2022, 2:42 a.m. UTC | #7

Hi Julien

Before submitting the v6 patch series, I would like to double confirm that ...

> -----Original Message-----
> From: Julien Grall <[e-mail]>
> Sent: Wednesday, June 29, 2022 6:18 PM
> To: Penny Zheng <[e-mail]>; [e-mail]
> Cc: Wei Chen <[e-mail]>; Stefano Stabellini
> <[e-mail]>; Bertrand Marquis <[e-mail]>;
> Volodymyr Babchuk <[e-mail]>
> Subject: Re: [PATCH v5 1/8] xen/arm: introduce static shared memory
> 
> 
> 
> On 29/06/2022 06:38, Penny Zheng wrote:
> > Hi Julien
> 
> Hi Penny,
> 
> >
> >> -----Original Message-----
> >> From: Julien Grall <[e-mail]>
> >> Sent: Saturday, June 25, 2022 1:55 AM
> >> To: Penny Zheng <[e-mail]>; xen-
> [e-mail]
> >> Cc: Wei Chen <[e-mail]>; Stefano Stabellini
> >> <[e-mail]>; Bertrand Marquis
> >> <[e-mail]>; Volodymyr Babchuk
> >> <[e-mail]>
> >> Subject: Re: [PATCH v5 1/8] xen/arm: introduce static shared memory
> >>
> >> Hi Penny,
> >>
> >> On 20/06/2022 06:11, Penny Zheng wrote:
> >>> From: Penny Zheng <[e-mail]>
> >>>
> >>> This patch serie introduces a new feature: setting up static
> >>
> >> Typo: s/serie/series/
> >>
> >>> shared memory on a dom0less system, through device tree configuration.
> >>>
> >>> This commit parses shared memory node at boot-time, and reserve it
> >>> in bootinfo.reserved_mem to avoid other use.
> >>>
> >>> This commits proposes a new Kconfig CONFIG_STATIC_SHM to wrap
> >>> static-shm-related codes, and this option depends on static memory(
> >>> CONFIG_STATIC_MEMORY). That's because that later we want to reuse a
> >>> few helpers, guarded with CONFIG_STATIC_MEMORY, like
> >>> acquire_staticmem_pages, etc, on static shared memory.
> >>>
> >>> Signed-off-by: Penny Zheng <[e-mail]>
> >>> Reviewed-by: Stefano Stabellini <[e-mail]>
> >>> ---
> >>> v5 change:
> >>> - no change
> >>> ---
> >>> v4 change:
> >>> - nit fix on doc
> >>> ---
> >>> v3 change:
> >>> - make nr_shm_domain unsigned int
> >>> ---
> >>> v2 change:
> >>> - document refinement
> >>> - remove bitmap and use the iteration to check
> >>> - add a new field nr_shm_domain to keep the number of shared domain
> >>> ---
> >>>    docs/misc/arm/device-tree/booting.txt | 120
> >> ++++++++++++++++++++++++++
> >>>    xen/arch/arm/Kconfig                  |   6 ++
> >>>    xen/arch/arm/bootfdt.c                |  68 +++++++++++++++
> >>>    xen/arch/arm/include/asm/setup.h      |   3 +
> >>>    4 files changed, 197 insertions(+)
> >>>
> >>> diff --git a/docs/misc/arm/device-tree/booting.txt
> >>> b/docs/misc/arm/device-tree/booting.txt
> >>> index 98253414b8..6467bc5a28 100644
> >>> --- a/docs/misc/arm/device-tree/booting.txt
> >>> +++ b/docs/misc/arm/device-tree/booting.txt
> >>> @@ -378,3 +378,123 @@ device-tree:
> >>>
> >>>    This will reserve a 512MB region starting at the host physical address
> >>>    0x30000000 to be exclusively used by DomU1.
> >>> +
> >>> +Static Shared Memory
> >>> +====================
> >>> +
> >>> +The static shared memory device tree nodes allow users to
> >>> +statically set up shared memory on dom0less system, enabling
> >>> +domains to do shm-based communication.
> >>> +
> >>> +- compatible
> >>> +
> >>> +    "xen,domain-shared-memory-v1"
> >>> +
> >>> +- xen,shm-id
> >>> +
> >>> +    An 8-bit integer that represents the unique identifier of the
> >>> + shared
> >> memory
> >>> +    region. The maximum identifier shall be "xen,shm-id = <0xff>".
> >>> +
> >>> +- xen,shared-mem
> >>> +
> >>> +    An array takes a physical address, which is the base address of the
> >>> +    shared memory region in host physical address space, a size,
> >>> + and a
> >> guest
> >>> +    physical address, as the target address of the mapping. The
> >>> + number of
> >> cells
> >>> +    for the host address (and size) is the same as the guest pseudo-
> physical
> >>> +    address and they are inherited from the parent node.
> >>
> >> Sorry for jump in the discussion late. But as this is going to be a
> >> stable ABI, I would to make sure the interface is going to be easily
> extendable.
> >>
> >> AFAIU, with your proposal the host physical address is mandatory. I
> >> would expect that some user may want to share memory but don't care
> >> about the exact location in memory. So I think it would be good to
> >> make it optional in the binding.
> >>
> >> I think this wants to be done now because it would be difficult to
> >> change the binding afterwards (the host physical address is the first set of
> cells).
> >>
> >> The Xen doesn't need to handle the optional case.
> >>

... what you suggested here is that during "xen,shared-mem" device tree property
parsing process, if we find that user doesn't provide physical address, we will output
an error, saying that it is not supported at the moment, right?

> >
> > Sure, I'll make "the host physical address" optional here, and right
> > now, with no actual code implementation. I'll make up it later in free
> > time~
> >
> > The user case you mentioned here is that we let xen to allocate an
> > arbitrary static shared memory region, so size and guest physical address
> are still mandatory, right?
> 
> That's correct.
> 
> Cheers,
> 
> --
> Julien Grall

Julien Grall July 13, 2022, 9:09 a.m. UTC | #8

Hi Penny,

On 13/07/2022 03:42, Penny Zheng wrote:
>>>>
>>>> The Xen doesn't need to handle the optional case.
>>>>
> 
> ... what you suggested here is that during "xen,shared-mem" device tree property
> parsing process, if we find that user doesn't provide physical address, we will output
> an error, saying that it is not supported at the moment, right?

You are correct.

Cheers,

Julien Grall July 15, 2022, 6:10 p.m. UTC | #9

Hi Penny,

On 29/06/2022 09:39, Penny Zheng wrote:
>>> +    for ( i = 0; i < mem->nr_banks; i++ )
>>> +    {
>>> +        /*
>>> +         * A static shared memory region could be shared between multiple
>>> +         * domains.
>>> +         */
>>> +        if ( paddr == mem->bank[i].start && size == mem->bank[i].size )
>>> +            break;
> 
> Maybe I need to add a check on shm-id:
> "
>          /*
>           * A static shared memory region could be shared between multiple
>           * domains.
>           */
>          if ( strcmp(shm_id, mem->bank[i].shm_id) == 0 )
>          {
>              if ( paddr == mem->bank[i].start && size == mem->bank[i].size )
>                  break;
>              else
>              {
>                  printk("Warning: xen,shm-id %s does not match for all the nodes using the same region.\n",
>                         shm_id);
>                  return -EINVAL;
>              }
>          }
> "
> Wdyt？

AFAICT, this would allow to region to overlap if they have different shm 
ID. I am not entirely sure the rest of your code would work properly in 
this case (what if the owner is different).

So I think we need the following checks:
   1) The shm ID matches *and* the region exactly match
   2) The shm ID doesn't match and the region doesn't overlap with an 
existing one

Cheers,

Penny Zheng July 18, 2022, 2:35 a.m. UTC | #10

Hi Julien

> -----Original Message-----
> From: Julien Grall <[e-mail]>
> Sent: Saturday, July 16, 2022 2:10 AM
> To: Penny Zheng <[e-mail]>; [e-mail]
> Cc: Wei Chen <[e-mail]>; Stefano Stabellini
> <[e-mail]>; Bertrand Marquis <[e-mail]>;
> Volodymyr Babchuk <[e-mail]>
> Subject: Re: [PATCH v5 1/8] xen/arm: introduce static shared memory
> 
> Hi Penny,
> 
> On 29/06/2022 09:39, Penny Zheng wrote:
> >>> +    for ( i = 0; i < mem->nr_banks; i++ )
> >>> +    {
> >>> +        /*
> >>> +         * A static shared memory region could be shared between multiple
> >>> +         * domains.
> >>> +         */
> >>> +        if ( paddr == mem->bank[i].start && size == mem->bank[i].size )
> >>> +            break;
> >
> > Maybe I need to add a check on shm-id:
> > "
> >          /*
> >           * A static shared memory region could be shared between multiple
> >           * domains.
> >           */
> >          if ( strcmp(shm_id, mem->bank[i].shm_id) == 0 )
> >          {
> >              if ( paddr == mem->bank[i].start && size == mem->bank[i].size )
> >                  break;
> >              else
> >              {
> >                  printk("Warning: xen,shm-id %s does not match for all the nodes
> using the same region.\n",
> >                         shm_id);
> >                  return -EINVAL;
> >              }
> >          }
> > "
> > Wdyt？
> 
> AFAICT, this would allow to region to overlap if they have different shm ID. I
> am not entirely sure the rest of your code would work properly in this case
> (what if the owner is different).
> 
> So I think we need the following checks:
>    1) The shm ID matches *and* the region exactly match
>    2) The shm ID doesn't match and the region doesn't overlap with an
> existing one
> 

Understood, true, the overlap shall also be checked.
"
@@ -451,6 +453,31 @@ static int __init process_shm_node(const void *fdt, int node,
                 return -EINVAL;
             }
         }
+        else
+        {
+            paddr_t end = paddr + size;
+            paddr_t bank_end = mem->bank[i].start + mem->bank[i].size;
+
+            if ( (paddr < mem->bank[i].start && end <= mem->bank[i].start) ||
+                 (paddr >= bank_end && end > bank_end) )
+            {
+                if ( strncmp(shm_id, mem->bank[i].shm_id,
+                             MAX_SHM_ID_LENGTH) != 0 )
+                    break;
+                else
+                {
+                    printk("fdt: different shared memory region could not share the same shm ID %s\n",
+                           shm_id);
+                    return -EINVAL;
+                }
+            }
+            else
+            {
+                printk("fdt: shared memory region overlap with an existing entry %#"PRIpaddr" - %#"PRIpaddr"\n",
+                        mem->bank[i].start, bank_end);
+                return -EINVAL;
+            }
+        }
     }

     if ( i == mem->nr_banks )
"

> Cheers,
> 
> --
> Julien Grall

Patch

diff --git a/docs/misc/arm/device-tree/booting.txt b/docs/misc/arm/device-tree/booting.txt
index 98253414b8..6467bc5a28 100644
--- a/docs/misc/arm/device-tree/booting.txt
+++ b/docs/misc/arm/device-tree/booting.txt
@@ -378,3 +378,123 @@  device-tree:
 
 This will reserve a 512MB region starting at the host physical address
 0x30000000 to be exclusively used by DomU1.
+
+Static Shared Memory
+====================
+
+The static shared memory device tree nodes allow users to statically set up
+shared memory on dom0less system, enabling domains to do shm-based
+communication.
+
+- compatible
+
+    "xen,domain-shared-memory-v1"
+
+- xen,shm-id
+
+    An 8-bit integer that represents the unique identifier of the shared memory
+    region. The maximum identifier shall be "xen,shm-id = <0xff>".
+
+- xen,shared-mem
+
+    An array takes a physical address, which is the base address of the
+    shared memory region in host physical address space, a size, and a guest
+    physical address, as the target address of the mapping. The number of cells
+    for the host address (and size) is the same as the guest pseudo-physical
+    address and they are inherited from the parent node.
+
+- role (Optional)
+
+    A string property specifying the ownership of a shared memory region,
+    the value must be one of the following: "owner", or "borrower"
+    A shared memory region could be explicitly backed by one domain, which is
+    called "owner domain", and all the other domains who are also sharing
+    this region are called "borrower domain".
+    If not specified, the default value is "borrower" and owner is
+    "dom_shared", a system domain.
+
+As an example:
+
+chosen {
+    #address-cells = <0x1>;
+    #size-cells = <0x1>;
+    xen,xen-bootargs = "console=dtuart dtuart=serial0 bootscrub=0";
+
+    ......
+
+    /* this is for Dom0 */
+    dom0-shared-mem@10000000 {
+        compatible = "xen,domain-shared-memory-v1";
+        role = "owner";
+        xen,shm-id = <0x0>;
+        xen,shared-mem = <0x10000000 0x10000000 0x10000000>;
+    }
+
+    domU1 {
+        compatible = "xen,domain";
+        #address-cells = <0x1>;
+        #size-cells = <0x1>;
+        memory = <0 131072>;
+        cpus = <2>;
+        vpl011;
+
+        /*
+         * shared memory region identified as 0x0(xen,shm-id = <0x0>)
+         * is shared between Dom0 and DomU1.
+         */
+        domU1-shared-mem@10000000 {
+            compatible = "xen,domain-shared-memory-v1";
+            role = "borrower";
+            xen,shm-id = <0x0>;
+            xen,shared-mem = <0x10000000 0x10000000 0x50000000>;
+        }
+
+        /*
+         * shared memory region identified as 0x1(xen,shm-id = <0x1>)
+         * is shared between DomU1 and DomU2.
+         */
+        domU1-shared-mem@50000000 {
+            compatible = "xen,domain-shared-memory-v1";
+            xen,shm-id = <0x1>;
+            xen,shared-mem = <0x50000000 0x20000000 0x60000000>;
+        }
+
+        ......
+
+    };
+
+    domU2 {
+        compatible = "xen,domain";
+        #address-cells = <0x1>;
+        #size-cells = <0x1>;
+        memory = <0 65536>;
+        cpus = <1>;
+
+        /*
+         * shared memory region identified as 0x1(xen,shm-id = <0x1>)
+         * is shared between domU1 and domU2.
+         */
+        domU2-shared-mem@50000000 {
+            compatible = "xen,domain-shared-memory-v1";
+            xen,shm-id = <0x1>;
+            xen,shared-mem = <0x50000000 0x20000000 0x70000000>;
+        }
+
+        ......
+    };
+};
+
+This is an example with two static shared memory regions.
+
+For the static shared memory region identified as 0x0, host physical
+address starting at 0x10000000 of 256MB will be reserved to be shared between
+Dom0 and DomU1. It will get mapped at 0x10000000 in Dom0 guest physical address
+space, and at 0x50000000 in DomU1 guest physical address space. Dom0 is
+explicitly defined as the owner domain, and DomU1 is the borrower domain.
+
+For the static shared memory region identified as 0x1, host physical
+address starting at 0x50000000 of 512MB will be reserved to be shared between
+DomU1 and DomU2. It will get mapped at 0x60000000 in DomU1 guest physical
+address space, and at 0x70000000 in DomU2 guest physical address space. DomU1
+and DomU2 are both the borrower domain, the owner domain is the default owner
+domain dom_shared.
diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
index be9eff0141..7321f47c0f 100644
--- a/xen/arch/arm/Kconfig
+++ b/xen/arch/arm/Kconfig
@@ -139,6 +139,12 @@  config TEE
 
 source "arch/arm/tee/Kconfig"
 
+config STATIC_SHM
+	bool "Statically shared memory on a dom0less system" if UNSUPPORTED
+	depends on STATIC_MEMORY
+	help
+	  This option enables statically shared memory on a dom0less system.
+
 endmenu
 
 menu "ARM errata workaround via the alternative framework"
diff --git a/xen/arch/arm/bootfdt.c b/xen/arch/arm/bootfdt.c
index ec81a45de9..38dcb05d5d 100644
--- a/xen/arch/arm/bootfdt.c
+++ b/xen/arch/arm/bootfdt.c
@@ -361,6 +361,70 @@  static int __init process_domain_node(const void *fdt, int node,
                                    size_cells, &bootinfo.reserved_mem, true);
 }
 
+#ifdef CONFIG_STATIC_SHM
+static int __init process_shm_node(const void *fdt, int node,
+                                   u32 address_cells, u32 size_cells)
+{
+    const struct fdt_property *prop;
+    const __be32 *cell;
+    paddr_t paddr, size;
+    struct meminfo *mem = &bootinfo.reserved_mem;
+    unsigned long i;
+
+    if ( address_cells < 1 || size_cells < 1 )
+    {
+        printk("fdt: invalid #address-cells or #size-cells for static shared memory node.\n");
+        return -EINVAL;
+    }
+
+    prop = fdt_get_property(fdt, node, "xen,shared-mem", NULL);
+    if ( !prop )
+        return -ENOENT;
+
+    /*
+     * xen,shared-mem = <paddr, size, gaddr>;
+     * Memory region starting from physical address #paddr of #size shall
+     * be mapped to guest physical address #gaddr as static shared memory
+     * region.
+     */
+    cell = (const __be32 *)prop->data;
+    device_tree_get_reg(&cell, address_cells, size_cells, &paddr, &size);
+    for ( i = 0; i < mem->nr_banks; i++ )
+    {
+        /*
+         * A static shared memory region could be shared between multiple
+         * domains.
+         */
+        if ( paddr == mem->bank[i].start && size == mem->bank[i].size )
+            break;
+    }
+
+    if ( i == mem->nr_banks )
+    {
+        if ( i < NR_MEM_BANKS )
+        {
+            /* Static shared memory shall be reserved from any other use. */
+            mem->bank[mem->nr_banks].start = paddr;
+            mem->bank[mem->nr_banks].size = size;
+            mem->bank[mem->nr_banks].xen_domain = true;
+            mem->nr_banks++;
+        }
+        else
+        {
+            printk("Warning: Max number of supported memory regions reached.\n");
+            return -ENOSPC;
+        }
+    }
+    /*
+     * keep a count of the number of domains, which later may be used to
+     * calculate the number of the reference count.
+     */
+    mem->bank[i].nr_shm_domain++;
+
+    return 0;
+}
+#endif
+
 static int __init early_scan_node(const void *fdt,
                                   int node, const char *name, int depth,
                                   u32 address_cells, u32 size_cells,
@@ -386,6 +450,10 @@  static int __init early_scan_node(const void *fdt,
         process_chosen_node(fdt, node, name, address_cells, size_cells);
     else if ( depth == 2 && device_tree_node_compatible(fdt, node, "xen,domain") )
         rc = process_domain_node(fdt, node, name, address_cells, size_cells);
+#ifdef CONFIG_STATIC_SHM
+    else if ( depth <= 3 && device_tree_node_compatible(fdt, node, "xen,domain-shared-memory-v1") )
+        rc = process_shm_node(fdt, node, address_cells, size_cells);
+#endif
 
     if ( rc < 0 )
         printk("fdt: node `%s': parsing failed\n", name);
diff --git a/xen/arch/arm/include/asm/setup.h b/xen/arch/arm/include/asm/setup.h
index 2bb01ecfa8..5063e5d077 100644
--- a/xen/arch/arm/include/asm/setup.h
+++ b/xen/arch/arm/include/asm/setup.h
@@ -27,6 +27,9 @@  struct membank {
     paddr_t start;
     paddr_t size;
     bool xen_domain; /* whether the memory bank is bound to a Xen domain. */
+#ifdef CONFIG_STATIC_SHM
+    unsigned int nr_shm_domain;
+#endif
 };
 
 struct meminfo {