On Tue, Apr 07, 2015 at 05:47:42PM -0700, Alexander Duyck wrote:
> This change makes it so that instead of using smp_wmb/rmb which varies
> depending on the kernel configuration we can can use dma_wmb/rmb which for
> most architectures should be equal to or slightly more strict than
> smp_wmb/rmb.
>
> The advantage to this is that these barriers are available to uniprocessor
> builds as well so the performance should improve under such a
> configuration.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxx>
Well the generic implementation has:
#ifndef dma_rmb
#define dma_rmb() rmb()
#endif
#ifndef dma_wmb
#define dma_wmb() wmb()
#endif
So for these arches you are slightly speeding up UP but slightly hurting SMP -
I think we did benchmark the difference as measureable in the past.
Additionally, isn't this relying on undocumented behaviour?
The documentation says:
"These are for use with consistent memory"
and virtio does not bother to request consistent memory
allocations.
One wonders whether these will always be strong enough.
> ---
> include/linux/virtio_ring.h | 23 ++++-------------------
> 1 file changed, 4 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> index 67e06fe18c03..8e50888a6d59 100644
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -21,19 +21,20 @@
> * actually quite cheap.
> */
>
> -#ifdef CONFIG_SMP
> static inline void virtio_mb(bool weak_barriers)
> {
> +#ifdef CONFIG_SMP
> if (weak_barriers)
> smp_mb();
> else
> +#endif
> mb();
> }
>
> static inline void virtio_rmb(bool weak_barriers)
> {
> if (weak_barriers)
> - smp_rmb();
> + dma_rmb();
> else
> rmb();
> }
> @@ -41,26 +42,10 @@ static inline void virtio_rmb(bool weak_barriers)
> static inline void virtio_wmb(bool weak_barriers)
> {
> if (weak_barriers)
> - smp_wmb();
> + dma_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;
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