linux-kernel - Re: [PATCH] Revert "vring: Force use of DMA API for ARM-based systems with legacy devices"

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Message-ID: <20170203095718.GB11585@arm.com>
Date:   Fri, 3 Feb 2017 09:57:19 +0000
From:   Will Deacon <will.deacon@....com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Robin Murphy <robin.murphy@....com>,
        stable@...r.kernel.org, Marc Zyngier <marc.zyngier@....com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] Revert "vring: Force use of DMA API for ARM-based
 systems with legacy devices"

On Fri, Feb 03, 2017 at 05:49:11AM +0200, Michael S. Tsirkin wrote:
> This reverts commit c7070619f3408d9a0dffbed9149e6f00479cf43b.
> 
> This has been shown to regress on some ARM systems:
> 
> by forcing on DMA API usage for ARM systems, we have inadvertently
> kicked open a hornets' nest in terms of cache-coherency. Namely that
> unless the virtio device is explicitly described as capable of coherent
> DMA by firmware, the DMA APIs on ARM and other DT-based platforms will
> assume it is non-coherent. This turns out to cause a big problem for the
> likes of QEMU and kvmtool, which generate virtio-mmio devices in their
> guest DTs but neglect to add the often-overlooked "dma-coherent"
> property; as a result, we end up with the guest making non-cacheable
> accesses to the vring, the host doing so cacheably, both talking past
> each other and things going horribly wrong.
> 
> We are working on a safer work-around.
> 
> Fixes: c7070619f340 ("vring: Force use of DMA API for ARM-based systems with legacy devices")
> Reported-by: Robin Murphy <robin.murphy@....com>
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Will Deacon <will.deacon@....com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> Acked-by: Marc Zyngier <marc.zyngier@....com>
> ---
> 
> I'll merge this for 4.10. Let's fix it properly for 4.11.

Acked-by: Will Deacon <will.deacon@....com>

Will