linux-kernel - Re: [PATCH v2 30/32] virtio_ring: update weak barriers to use __smp

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Date:	Fri, 1 Jan 2016 12:21:52 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	virtualization@...ts.linux-foundation.org,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	David Miller <davem@...emloft.net>, linux-ia64@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-metag@...r.kernel.org, linux-mips@...ux-mips.org,
	x86@...nel.org, user-mode-linux-devel@...ts.sourceforge.net,
	adi-buildroot-devel@...ts.sourceforge.net,
	linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	xen-devel@...ts.xenproject.org,
	Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH v2 30/32] virtio_ring: update weak barriers to use
 __smp_xxx

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@...radead.org>
> Cc: Alexander Duyck <alexander.duyck@...il.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

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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