linux-kernel - Re: [PATCH v2 32/32] virtio_ring: use virt_store

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Date:	Fri, 1 Jan 2016 20:23:46 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>, 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
Subject: Re: [PATCH v2 32/32] virtio_ring: use virt_store_mb

Hello.

On 12/31/2015 10:09 PM, Michael S. Tsirkin wrote:

> We need a full barrier after writing out event index, using
> virt_store_mb there seems better than open-coding.  As usual, we need a
> wrapper to account for strong barriers.
>
> It's tempting to use this in vhost as well, for that, we'll
> need a variant of smp_store_mb that works on __user pointers.
>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>   include/linux/virtio_ring.h  | 12 ++++++++++++
>   drivers/virtio/virtio_ring.c | 15 +++++++++------
>   2 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> index f3fa55b..3a74d91 100644
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -45,6 +45,18 @@ static inline void virtio_wmb(bool weak_barriers)
>   		wmb();
>   }
>
> +static inline void virtio_store_mb(bool weak_barriers,
> +				   __virtio16 *p, __virtio16 v)
> +{
> +	if (weak_barriers)
> +		virt_store_mb(*p, v);
> +	else
> +	{

    The kernel coding style dictates:

	if (weak_barriers) {
		virt_store_mb(*p, v);
	} else {

> +		WRITE_ONCE(*p, v);
> +		mb();
> +	}
> +}
> +
[...]

MBR, Sergei

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