linux-kernel - Re: [PATCH 2/3] virtio

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Date:	Tue, 27 Oct 2015 22:12:23 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Joerg Roedel <jroedel@...e.de>
Cc:	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Christoph Hellwig <hch@....de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	KVM <kvm@...r.kernel.org>, David Woodhouse <dwmw2@...radead.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: [PATCH 2/3] virtio_ring: Support DMA APIs

On Tue, Oct 27, 2015 at 7:21 PM, Joerg Roedel <jroedel@...e.de> wrote:
> On Tue, Oct 27, 2015 at 07:13:56PM -0700, Andy Lutomirski wrote:
>> On Tue, Oct 27, 2015 at 7:06 PM, Joerg Roedel <jroedel@...e.de> wrote:
>> > Hi Andy,
>> >
>> > On Tue, Oct 27, 2015 at 06:17:09PM -0700, Andy Lutomirski wrote:
>> >> From: Andy Lutomirski <luto@...capital.net>
>> >>
>> >> virtio_ring currently sends the device (usually a hypervisor)
>> >> physical addresses of its I/O buffers.  This is okay when DMA
>> >> addresses and physical addresses are the same thing, but this isn't
>> >> always the case.  For example, this never works on Xen guests, and
>> >> it is likely to fail if a physical "virtio" device ever ends up
>> >> behind an IOMMU or swiotlb.
>> >
>> > The overall code looks good, but I havn't seen and dma_sync* calls.
>> > When swiotlb=force is in use this would break.
>> >
>> >> +             vq->vring.desc[head].addr = cpu_to_virtio64(_vq->vdev, vring_map_single(
>> >> +                     vq,
>> >> +                     desc, total_sg * sizeof(struct vring_desc),
>> >> +                     DMA_TO_DEVICE));
>> >
>>
>> Are you talking about a dma_sync call on the descriptor ring itself?
>> Isn't dma_alloc_coherent supposed to make that unnecessary?  I should
>> move the allocation into the virtqueue code.
>>
>> The docs suggest that I might need to "flush the processor's write
>> buffers before telling devices to read that memory".  I'm not sure how
>> to do that.
>
> The write buffers should be flushed by the dma-api functions if
> necessary.  For dma_alloc_coherent allocations you don't need to call
> dma_sync*, but for the map_single/map_page/map_sg ones, as these might
> be bounce-buffered.

I think that all the necessary barriers are already there.  I had a
nasty bug that swiotlb=force exposed, though, which I've fixed.

--Andy
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