linux-kernel - Re: [PATCH] virtio_ring: switch to dma

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Message-ID: <5047d08c-9f9e-2902-d425-1fac849ce5b4@redhat.com>
Date:   Thu, 19 Apr 2018 19:39:21 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Ohad Ben-Cohen <ohad@...ery.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-remoteproc@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio_ring: switch to dma_XX barriers for rpmsg

On 19/04/2018 19:35, Michael S. Tsirkin wrote:
> virtio is using barriers to order memory accesses, thus
> dma_wmb/rmb is a good match.
> 
> Build-tested on x86: Before
> 
> [mst@...k linux]$ size drivers/virtio/virtio_ring.o
>    text    data     bss     dec     hex filename
>   11392     820       0   12212    2fb4 drivers/virtio/virtio_ring.o
> 
> After
> mst@...k linux]$ size drivers/virtio/virtio_ring.o
>    text    data     bss     dec     hex filename
>   11284     820       0   12104    2f48 drivers/virtio/virtio_ring.o
> 
> Cc: Ohad Ben-Cohen <ohad@...ery.com>
> Cc: Bjorn Andersson <bjorn.andersson@...aro.org>
> Cc: linux-remoteproc@...r.kernel.org
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
> 
> It's good in theory, but could one of RPMSG maintainers please review
> and ack this patch? Or even better test it?
> 
> All these barriers are useless on Intel anyway ...

This should be okay, but I wonder if there should be a virtio_wmb(...)
or an "if (weak_barriers) wmb()" before the "writel" in vm_notify
(drivers/virtio/virtio_mmio.c).

Thanks,

Paolo

> 
>  include/linux/virtio_ring.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> index bbf3252..fab0213 100644
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -35,7 +35,7 @@ static inline void virtio_rmb(bool weak_barriers)
>  	if (weak_barriers)
>  		virt_rmb();
>  	else
> -		rmb();
> +		dma_rmb();
>  }
>  
>  static inline void virtio_wmb(bool weak_barriers)
> @@ -43,7 +43,7 @@ static inline void virtio_wmb(bool weak_barriers)
>  	if (weak_barriers)
>  		virt_wmb();
>  	else
> -		wmb();
> +		dma_wmb();
>  }
>  
>  static inline void virtio_store_mb(bool weak_barriers,
>