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

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Date:	Wed, 28 Oct 2015 11:06:50 +0900
From:	Joerg Roedel <jroedel@...e.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	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>, benh@...nel.crashing.org,
	KVM <kvm@...r.kernel.org>, dwmw2@...radead.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 2/3] virtio_ring: Support DMA APIs

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));

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


	Joerg

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