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

[Michael S. Tsirkin]

On Thu, Oct 11, 2012 at 03:44:10PM +0200, Avi Kivity wrote:
> 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 <address@hidden>
> >> > 
> >> > 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.

Okay so there will be some API that virtio devices should call
to achieve this?

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