diff mbox series

[2/2] spapr: Enable virtio iommu_platform=on by default

Message ID 20200207043055.218856-3-david@gibson.dropbear.id.au
State New, archived
Headers show
Series spapr: Use vIOMMU translation for virtio by default | expand

Commit Message

David Gibson Feb. 7, 2020, 4:30 a.m. UTC
Traditionally, virtio devices don't do DMA by the usual path on the
guest platform.  In particular they usually bypass any virtual IOMMU
the guest has, using hypervisor magic to access untranslated guest
physical addresses.

There's now the optional iommu_platform flag which can tell virtio
devices to use the platform's normal DMA path, including any IOMMUs.
That flag was motiviated for the case of hardware virtio
implementations, but there are other reasons to want it.

Specifically, the fact that the virtio device doesn't use vIOMMU
translation means that virtio devices are unsafe to pass to nested
guests, or to use with VFIO userspace drivers inside the guest.  This
is particularly noticeable on the pseries platform which *always* has
a guest-visible vIOMMU.

Not using the normal DMA path also causes difficulties for the guest
side driver when using the upcoming POWER Secure VMs (a.k.a. PEF).
While it's theoretically possible to handle this on the guest side,
it's really fiddly.  Given the other problems with the non-translated
virtio device, let's just enable vIOMMU translation for virtio devices
by default in the pseries-5.0 (and later) machine types.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 hw/ppc/spapr.c | 2 ++
 1 file changed, 2 insertions(+)

Comments

Michael S. Tsirkin Feb. 7, 2020, 6:57 a.m. UTC | #1
On Fri, Feb 07, 2020 at 03:30:55PM +1100, David Gibson wrote:
> Traditionally, virtio devices don't do DMA by the usual path on the
> guest platform.  In particular they usually bypass any virtual IOMMU
> the guest has, using hypervisor magic to access untranslated guest
> physical addresses.
> 
> There's now the optional iommu_platform flag which can tell virtio
> devices to use the platform's normal DMA path, including any IOMMUs.
> That flag was motiviated for the case of hardware virtio
> implementations, but there are other reasons to want it.
> 
> Specifically, the fact that the virtio device doesn't use vIOMMU
> translation means that virtio devices are unsafe to pass to nested
> guests, or to use with VFIO userspace drivers inside the guest.  This
> is particularly noticeable on the pseries platform which *always* has
> a guest-visible vIOMMU.
> 
> Not using the normal DMA path also causes difficulties for the guest
> side driver when using the upcoming POWER Secure VMs (a.k.a. PEF).
> While it's theoretically possible to handle this on the guest side,
> it's really fiddly.  Given the other problems with the non-translated
> virtio device, let's just enable vIOMMU translation for virtio devices
> by default in the pseries-5.0 (and later) machine types.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Worth noting that since iommu_platform is mandatory for guests,
this disables support for guests older than Linux 4.8.


> ---
>  hw/ppc/spapr.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 216d3b34dc..78e031e80a 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -4518,6 +4518,7 @@ static void spapr_machine_5_0_class_options(MachineClass *mc)
>       * default behaviour for virtio */
>      static GlobalProperty compat[] = {
>          { TYPE_VIRTIO_PCI, "disable-legacy", "on", },
> +        { TYPE_VIRTIO_DEVICE, "iommu_platform", "on", },
>      };
>  
>      compat_props_add(mc->compat_props, compat, G_N_ELEMENTS(compat));
> @@ -4533,6 +4534,7 @@ static void spapr_machine_4_2_class_options(MachineClass *mc)
>      SpaprMachineClass *smc = SPAPR_MACHINE_CLASS(mc);
>      static GlobalProperty compat[] = {
>          { TYPE_VIRTIO_PCI, "disable-legacy", "auto" },
> +        { TYPE_VIRTIO_DEVICE, "iommu_platform", "off", },
>      };
>  
>      spapr_machine_5_0_class_options(mc);
> -- 
> 2.24.1
diff mbox series

Patch

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 216d3b34dc..78e031e80a 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -4518,6 +4518,7 @@  static void spapr_machine_5_0_class_options(MachineClass *mc)
      * default behaviour for virtio */
     static GlobalProperty compat[] = {
         { TYPE_VIRTIO_PCI, "disable-legacy", "on", },
+        { TYPE_VIRTIO_DEVICE, "iommu_platform", "on", },
     };
 
     compat_props_add(mc->compat_props, compat, G_N_ELEMENTS(compat));
@@ -4533,6 +4534,7 @@  static void spapr_machine_4_2_class_options(MachineClass *mc)
     SpaprMachineClass *smc = SPAPR_MACHINE_CLASS(mc);
     static GlobalProperty compat[] = {
         { TYPE_VIRTIO_PCI, "disable-legacy", "auto" },
+        { TYPE_VIRTIO_DEVICE, "iommu_platform", "off", },
     };
 
     spapr_machine_5_0_class_options(mc);