Re: [Qemu-devel] [RFC PATCH] vfio: VFIO PCI driver for Qemu

[Alex Williamson]

On Thu, 2012-07-26 at 19:06 +0300, Avi Kivity wrote:
> On 07/26/2012 05:56 PM, Alex Williamson wrote:
> > 
> > Both KVM and VFIO do strive to make the device in the guest look as much
> > like it does on bare metal as possible, but we don't guarantee they're
> > identical and we don't guarantee to match each other.
> 
> btw, this is somewhat problematic, conceivably this could break a guest
> (due to a guest bug).  But with device assignment the compatibility
> requirements can be relaxed a bit since there is no live migration.

Well, I would hope that things work better in vfio and we work to make
that the recommended method of device assignment.  We can't hold one
back to make things identical.  The only barrier I see to this is that
vfio focuses on security, enforcing things like ACS to make sure devices
can't do DMA to other devices outside of the group whereas KVM
assignment will let you attempt to do nearly anything and counts on
libvirt to only let the user attempt to do sane things.  As you say,
there's no live migration with device assignment, so absolute identical
config space is not a requirement and the difference we do have should
be sufficiently subtle that the guest doesn't care boot-to-boot.
Thanks,

Alex