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Re: [Qemu-devel] [PATCH V2 RFC] fixup! virtio: convert to use DMA api

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH V2 RFC] fixup! virtio: convert to use DMA api
Date: Wed, 27 Apr 2016 18:09:31 +0300

On Wed, Apr 27, 2016 at 04:58:51PM +0200, Joerg Roedel wrote:
> On Wed, Apr 27, 2016 at 05:54:57PM +0300, Michael S. Tsirkin wrote:
> > Point is, QEMU is not the only virtio implementation out there.
> > So we can't know no virtio implementations have an IOMMU as long as
> > linux supports this IOMMU.
> > virtio always used physical addresses since it was born and if it
> > changes that it must do this in a way that does not break existing
> > users.
> FWIW, virtio in qemu can continue to just use physical addresses. But
> qemu needs to advertise that fact correctly to the OS in the DMAR table.
> This way old kernels (where virtio does not use DMA-API) will also
> continue to work on the fixed qemu.
>       Joerg

It's not clear it can do this since DMAR tables seem to assume
a given slot is either bypassing IOMMU or going through it.
QEMU allows reusing same slot for virtio and non-virtio devices.

Besides, this patch is not about it, it's a flag for QEMU to
tell guest that it can trust DMAR tables.


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