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Date: Thu, 11 Oct 2012 15:44:28 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [RFC v1 7/7] vhost: abort if an emulated iommu is
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To: Avi Kivity <avi@redhat.com>
Cc: qemu-devel@nongnu.org, liu ping fan <qemulist@gmail.com>, Blue Swirl <blauwirbel@gmail.com>, Alex Williamson <alex.williamson@redhat.com>, Anthony Liguori <anthony@codemonkey.ws>, Paolo Bonzini <pbonzini@redhat.com>

On Thu, Oct 11, 2012 at 03:34:54PM +0200, Avi Kivity wrote:
> On 10/11/2012 03:31 PM, Michael S. Tsirkin wrote:
> > On Thu, Oct 11, 2012 at 03:27:03PM +0200, Avi Kivity wrote:
> >> vhost doesn't support guest iommus yet, indicate it to the user
> >> by gently depositing a core on their disk.
> >> 
> >> Signed-off-by: Avi Kivity <avi@redhat.com>
> > 
> > Actually there is no problem. virtio bypasses an IOMMU,
> > so vhost works fine by writing into guest memory directly.
> > 
> > So I don't think we need this patch.
> 
> The pci subsystem should set up the iommu so that it ignores virtio
> devices.  If it does, an emulated iommu will not reach vhost.  If it
> doesn't, then it will, and the assert() will alert us that we have a bug.

You mean pci subsystem in the guest? I'm pretty sure that's not
the case at the moment: iommu is on by default and applies
to all devices unless you do something special.
I see where you are coming from but it does
not look right to break all existing guests.

Also - I see no reason to single out vhost - I think same applies with
any virtio device, since it doesn't use the DMA API.

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