Re: [Xen-devel] [PATCH v2 30/32] virtio_ring: update weak barriers to use __smp_xxx

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Fri, Jan 01, 2016 at 11:39:40AM +0200, Michael S. Tsirkin wrote:
> virtio ring uses smp_wmb on SMP and wmb on !SMP,
> the reason for the later being that it might be
> talking to another kernel on the same SMP machine.
> 
> This is exactly what __smp_XXX barriers do,
> so switch to these instead of homegrown ifdef hacks.
> 
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Alexander Duyck <alexander.duyck@xxxxxxxxx>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

The subject and commit log should say
virt_xxx and not __smp_xxx - I fixed this up in
my tree.

> ---
>  include/linux/virtio_ring.h | 25 ++++---------------------
>  1 file changed, 4 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> index 67e06fe..f3fa55b 100644
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -12,7 +12,7 @@
>   * anyone care?
>   *
>   * For virtio_pci on SMP, we don't need to order with respect to MMIO
> - * accesses through relaxed memory I/O windows, so smp_mb() et al are
> + * accesses through relaxed memory I/O windows, so virt_mb() et al are
>   * sufficient.
>   *
>   * For using virtio to talk to real devices (eg. other heterogeneous
> @@ -21,11 +21,10 @@
>   * actually quite cheap.
>   */
>  
> -#ifdef CONFIG_SMP
>  static inline void virtio_mb(bool weak_barriers)
>  {
>       if (weak_barriers)
> -             smp_mb();
> +             virt_mb();
>       else
>               mb();
>  }
> @@ -33,7 +32,7 @@ static inline void virtio_mb(bool weak_barriers)
>  static inline void virtio_rmb(bool weak_barriers)
>  {
>       if (weak_barriers)
> -             smp_rmb();
> +             virt_rmb();
>       else
>               rmb();
>  }
> @@ -41,26 +40,10 @@ static inline void virtio_rmb(bool weak_barriers)
>  static inline void virtio_wmb(bool weak_barriers)
>  {
>       if (weak_barriers)
> -             smp_wmb();
> +             virt_wmb();
>       else
>               wmb();
>  }
> -#else
> -static inline void virtio_mb(bool weak_barriers)
> -{
> -     mb();
> -}
> -
> -static inline void virtio_rmb(bool weak_barriers)
> -{
> -     rmb();
> -}
> -
> -static inline void virtio_wmb(bool weak_barriers)
> -{
> -     wmb();
> -}
> -#endif
>  
>  struct virtio_device;
>  struct virtqueue;
> -- 
> MST

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