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From: Rusty Russell <rusty@rustcorp.com.au>
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Date: Wed, 19 Oct 2011 11:01:12 +1030
Message-ID: <87zkgxakkv.fsf@rustcorp.com.au>
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Subject: Re: [Qemu-devel] [PATCH 1/9] Add stub functions for PCI device
	models to do PCI DMA
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To: David Gibson <dwg@au1.ibm.com>, Avi Kivity <avi@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, "Michael S. Tsirkin" <mst@redhat.com>, joerg.roedel@amd.com, qemu-devel@nongnu.org, agraf@suse.de, kraxel@redhat.com, eduard.munteanu@linux360.ro, rth@twiddle.net

On Tue, 18 Oct 2011 12:46:50 +1100, David Gibson <dwg@au1.ibm.com> wrote:
> On Sun, Oct 16, 2011 at 03:15:53PM +0200, Avi Kivity wrote:
> > 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.
> 
> So, I just had a chat with Rusty about this.  Perhaps it shouldn't,
> but it does.  The spec is in terms of guest physical addresses, not
> bus/DMA addresses, and more to the point the Linux driver does *not*
> do the necessary dma_map() and unmap operations to treat this as a PCI
> DMA.  So like it or not, god-like hypervisor access rather than
> emulated PCI DMA is what it does.

Yep, it shouldn't but it does.  Can't break it now without a feature
bit, and there's no particular reason to add it until someone really
wants it.

Cheers,
Rusty.