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

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Date:	Tue, 27 Oct 2015 19:13:56 -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 7:06 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.

> Nit: I think readability could be improved here by using a temporary
> variable for the return value of vring_map_single().
>

I'll do something like that for v2.

--Andy
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