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

[Avi Kivity <avi@redhat.com>]

On 07/26/2012 07:40 PM, Alex Williamson wrote:
> 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.

We could add a strict backward compatibility option that forces the
layout, but it isn't worth it.

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