Re: [Qemu-devel] [RFC v1 7/7] vhost: abort if an emulated iommu is used

[Avi Kivity <avi@redhat.com>]

On 10/11/2012 03:44 PM, Michael S. Tsirkin wrote:
> 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.

No, qemu should configure virtio devices to bypass the iommu, even if it
is on.

> 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.

True.

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