Re: [PATCH v2 33/34] xenbus: use virt_xxx barriers

[Stefano Stabellini]

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 

Reviewed-by: Stefano Stabellini 

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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Re: [PATCH v2 33/34] xenbus: use virt_xxx barriers

[Peter Zijlstra]

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 
> ---
>  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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