[libvirt] [RFC PATCH 00/28] Enable multifunction pci hotplug

[Daniel P. Berrangé]

On Thu, Mar 15, 2018 at 09:54:46AM -0600, Alex Williamson wrote:
> On Thu, 15 Mar 2018 15:06:38 +0000
> Daniel P. Berrangé <berrange at redhat.com> wrote:
> > On Thu, Mar 15, 2018 at 08:59:41AM -0600, Alex Williamson wrote:
> > > 
> > > Neither really bothers me, but I'm confused by the claimed existing
> > > handling of SR-IOV.  Either you're assigning a PF and SR-IOV is
> > > irrelevant and unavailable to the guest or you're assigning a VF and,
> > > well, SR-IOV is still mostly irrelevant to libvirt unless someone
> > > decides to assign the PF hosting the VF or libvirt needs to do VF
> > > configuration via the PF.  Thanks,  
> > 
> > Hmm, could be a mis-understanding then. I was under the belief that
> > when you assign the PF or a SRIOV device to the guest, all the
> > VFs obviously disappear from the host due to driver being unloaded.
> > The guest now has the PF, and would have the option to enable VFs
> > too if it so desired, just as the host had option for when it owned
> > the PF.
> 
> Yeah, that's not how it currently works.  Some people would like it if
> this were the case, but we've not gotten past the security issues.  If
> the user is allowed to enable SR-IOV, those VFs don't just appear for
> the VM, they appear on the host.  The host needs to probe for them,
> assign resources, and attach drivers.  What should the host do with VFs
> that are managed by an untrusted userspace driver?  The isolation
> between VFs and PFs depends on the vendor's SR-IOV implementation.
> Minimally, the PF driver manages the PCI bus master config bit and can
> trivially introduce a denial of service for the VFs.  Allowing a VM to
> enable SR-IOV only for the purpose of assigning those VFs back to the VM
> owning the PF doesn't seem to be a particularly compelling feature on
> its own.  Thanks,

So it sounds like if, in the future, we did want to allow the guest to
have the PF, *and* all the VFs at the same time, we would probably need
to arrange that all from the host side, vaguely like is being proposed
in this series for non-SRIOV, and not let the guest have control over VF
create/delete itself. 

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|