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Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to


From: Avi Kivity
Subject: Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device models to do PCI DMA
Date: Sun, 16 Oct 2011 15:15:53 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:7.0) Gecko/20110927 Thunderbird/7.0

On 10/14/2011 04:14 AM, David Gibson wrote:
> > Virtio is a very, very special case.  virtio requires coherent RAM access.
>
> Right.  Virtio's access to memory is *not* emulated PCI DMA, it's
> god-like hypervisor access to guest system memory.  It should
> correctly bypass any IOMMU, and so should remain as
> cpu_physical_memory_rw() or the atomic accessors, rather than being
> converted to this new API.

virtio should definitely not bypass an iommu.  A guest may assign a
virtio device to nested guests, and would wish it confined by the
emulated iommu.

More generally, a guest sees a virtio device as just another pci device,
and has no way to tell that it bypasses the iommu.

-- 
error compiling committee.c: too many arguments to function




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