linux-kernel - Re: [Xen-devel] [PATCH RFC 0/3] Xen on Virtio

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Date:	Mon, 14 Dec 2015 16:12:13 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [Xen-devel] [PATCH RFC 0/3] Xen on Virtio

On Mon, Dec 14, 2015 at 02:00:05PM +0000, David Vrabel wrote:
> On 07/12/15 16:19, Stefano Stabellini wrote:
> > Hi all,
> > 
> > this patch series introduces support for running Linux on top of Xen
> > inside a virtual machine with virtio devices (nested virt scenario).
> > The problem is that Linux virtio drivers use virt_to_phys to get the
> > guest pseudo-physical addresses to pass to the backend, which doesn't
> > work as expected on Xen.
> > 
> > Switching the virtio drivers to the dma APIs (dma_alloc_coherent,
> > dma_map/unmap_single and dma_map/unmap_sg) would solve the problem, as
> > Xen support in Linux provides an implementation of the dma API which
> > takes care of the additional address conversions. However using the dma
> > API would increase the complexity of the non-Xen case too. We would also
> > need to keep track of the physical or virtual address in addition to the
> > dma address for each vring_desc to be able to free the memory in
> > detach_buf (see patch #3).
> > 
> > Instead this series adds few obvious checks to perform address
> > translations in a couple of key places, without changing non-Xen code
> > paths. You are welcome to suggest improvements or alternative
> > implementations.
> 
> Andy Lutomirski also looked at this.  Andy what happened to this work?
> 
> David

The approach there was to try and convert all virtio to use DMA
API unconditionally.
This is reasonable if there's a way for devices to request
1:1 mappings individually.
As that is currently missing, that patchset can not be merged yet.

-- 
MST
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