diff mbox

[v2,33/34] xenbus: use virt_xxx barriers

Message ID 1451572003-2440-34-git-send-email-mst@redhat.com (mailing list archive)
State Awaiting Upstream
Delegated to: Geert Uytterhoeven
Headers show

Commit Message

Michael S. Tsirkin Dec. 31, 2015, 7:10 p.m. UTC
drivers/xen/xenbus/xenbus_comms.c uses
full memory barriers to communicate with the other side.

For guests compiled with CONFIG_SMP, smp_wmb and smp_mb
would be sufficient, so mb() and wmb() here are only needed if
a non-SMP guest runs on an SMP host.

Switch to virt_xxx barriers which serve this exact purpose.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/xen/xenbus/xenbus_comms.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Comments

David Vrabel Jan. 4, 2016, 11:32 a.m. UTC | #1
On 31/12/15 19:10, Michael S. Tsirkin wrote:
> drivers/xen/xenbus/xenbus_comms.c uses
> full memory barriers to communicate with the other side.
> 
> For guests compiled with CONFIG_SMP, smp_wmb and smp_mb
> would be sufficient, so mb() and wmb() here are only needed if
> a non-SMP guest runs on an SMP host.
> 
> Switch to virt_xxx barriers which serve this exact purpose.

Acked-by: David Vrabel <david.vrabel@citrix.com>

If you're feeling particularly keen there's a rmb() consume_one_event()
in drivers/xen/events/events_fifo.c that can be converted to virt_rmb()
as well.

David
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Stefano Stabellini Jan. 4, 2016, 12:03 p.m. UTC | #2
On Thu, 31 Dec 2015, Michael S. Tsirkin wrote:
> drivers/xen/xenbus/xenbus_comms.c uses
> full memory barriers to communicate with the other side.
> 
> For guests compiled with CONFIG_SMP, smp_wmb and smp_mb
> would be sufficient, so mb() and wmb() here are only needed if
> a non-SMP guest runs on an SMP host.
> 
> Switch to virt_xxx barriers which serve this exact purpose.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

Are you also going to take care of

drivers/xen/grant-table.c
drivers/xen/evtchn.c
drivers/xen/events/events_fifo.c
drivers/xen/xen-scsiback.c
drivers/xen/tmem.c
drivers/xen/xen-pciback/pci_stub.c
drivers/xen/xen-pciback/pciback_ops.c

?


>  drivers/xen/xenbus/xenbus_comms.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
> index fdb0f33..ecdecce 100644
> --- a/drivers/xen/xenbus/xenbus_comms.c
> +++ b/drivers/xen/xenbus/xenbus_comms.c
> @@ -123,14 +123,14 @@ int xb_write(const void *data, unsigned len)
>  			avail = len;
>  
>  		/* Must write data /after/ reading the consumer index. */
> -		mb();
> +		virt_mb();
>  
>  		memcpy(dst, data, avail);
>  		data += avail;
>  		len -= avail;
>  
>  		/* Other side must not see new producer until data is there. */
> -		wmb();
> +		virt_wmb();
>  		intf->req_prod += avail;
>  
>  		/* Implies mb(): other side will see the updated producer. */
> @@ -180,14 +180,14 @@ int xb_read(void *data, unsigned len)
>  			avail = len;
>  
>  		/* Must read data /after/ reading the producer index. */
> -		rmb();
> +		virt_rmb();
>  
>  		memcpy(data, src, avail);
>  		data += avail;
>  		len -= avail;
>  
>  		/* Other side must not see free space until we've copied out */
> -		mb();
> +		virt_mb();
>  		intf->rsp_cons += avail;
>  
>  		pr_debug("Finished read of %i bytes (%i to go)\n", avail, len);
> -- 
> MST
> 
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Peter Zijlstra Jan. 4, 2016, 2:09 p.m. UTC | #3
On Thu, Dec 31, 2015 at 09:10:01PM +0200, Michael S. Tsirkin wrote:
> drivers/xen/xenbus/xenbus_comms.c uses
> full memory barriers to communicate with the other side.
> 
> For guests compiled with CONFIG_SMP, smp_wmb and smp_mb
> would be sufficient, so mb() and wmb() here are only needed if
> a non-SMP guest runs on an SMP host.
> 
> Switch to virt_xxx barriers which serve this exact purpose.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  drivers/xen/xenbus/xenbus_comms.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
> index fdb0f33..ecdecce 100644
> --- a/drivers/xen/xenbus/xenbus_comms.c
> +++ b/drivers/xen/xenbus/xenbus_comms.c
> @@ -123,14 +123,14 @@ int xb_write(const void *data, unsigned len)
>  			avail = len;
>  
>  		/* Must write data /after/ reading the consumer index. */
> -		mb();
> +		virt_mb();
>  

So its possible to remove this barrier entirely, see the "CONTROL
DEPENDNCIES" chunk of memory-barrier.txt.

But do that in a separate patch series and only if you really really
need the performance.
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diff mbox

Patch

diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
index fdb0f33..ecdecce 100644
--- a/drivers/xen/xenbus/xenbus_comms.c
+++ b/drivers/xen/xenbus/xenbus_comms.c
@@ -123,14 +123,14 @@  int xb_write(const void *data, unsigned len)
 			avail = len;
 
 		/* Must write data /after/ reading the consumer index. */
-		mb();
+		virt_mb();
 
 		memcpy(dst, data, avail);
 		data += avail;
 		len -= avail;
 
 		/* Other side must not see new producer until data is there. */
-		wmb();
+		virt_wmb();
 		intf->req_prod += avail;
 
 		/* Implies mb(): other side will see the updated producer. */
@@ -180,14 +180,14 @@  int xb_read(void *data, unsigned len)
 			avail = len;
 
 		/* Must read data /after/ reading the producer index. */
-		rmb();
+		virt_rmb();
 
 		memcpy(data, src, avail);
 		data += avail;
 		len -= avail;
 
 		/* Other side must not see free space until we've copied out */
-		mb();
+		virt_mb();
 		intf->rsp_cons += avail;
 
 		pr_debug("Finished read of %i bytes (%i to go)\n", avail, len);