Re: ovmf / PCI passthrough impaired due to very limiting PCI64 aperture

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Tue, Jun 16, 2020 at 01:10:21PM -0400, Eduardo Habkost wrote:
> > On Tue, Jun 16, 2020 at 05:57:46PM +0100, Dr. David Alan Gilbert wrote:
> > > * Gerd Hoffmann (kraxel@redhat.com) wrote:
> > > >   Hi,
> > > > 
> > > > > (a) We could rely in the guest physbits to calculate the PCI64 aperture.
> > > > 
> > > > I'd love to do that.  Move the 64-bit I/O window as high as possible and
> > > > use -- say -- 25% of the physical address space for it.
> > > > 
> > > > Problem is we can't.
> > > > 
> > > > > failure. Also, if the users are not setting the physbits in the guest,
> > > > > there must be a default (seems to be 40bit according to my experiments),
> > > > > seems to be a good idea to rely on that.
> > > > 
> > > > Yes, 40 is the default, and it is used *even if the host supports less
> > > > than that*.  Typical values I've seen for intel hardware are 36 and 39.
> > > > 39 is used even by recent hardware (not the xeons, but check out a
> > > > laptop or a nuc).
> > > > 
> > > > > If guest physbits is 40, why to have OVMF limiting it to 36, right?
> > > > 
> > > > Things will explode in case OVMF uses more physbits than the host
> > > > supports (host physbits limit applies to ept too).  In other words: OVMF
> > > > can't trust the guest physbits, so it is conservative to be on the safe
> > > > side.
> > > > 
> > > > If we can somehow make a *trustable* physbits value available to the
> > > > guest, then yes, we can go that route.  But the guest physbits we have
> > > > today unfortunately don't cut it.
> > > 
> > > In downstream RH qemu, we run with host-physbits as default; so it's reasonably
> > > trustworthy; of course that doesn't help you across a migration between
> > > hosts with different sizes (e.g. an E5 Xeon to an E3).
> > > Changing upstream to do the same would seem sensible to me, but it's not
> > > a foolproof config.
> > 
> > Yeah, to make it really trustworthy we would need to prevent
> > migration to hosts with mismatching phys sizes.  We would need to
> > communicate that to the guest somehow (with new hypervisor CPUID
> > flags, maybe).
> 
> QEMU should be able to validate the hostphysbits >= guestphysbits when
> accepting incoming migration, and abort it.

Yeh, there's an outstanding request to validate other CPU flags as well.

> Meanwhile libvirt should be enhanced to report hostphysbits, so that
> management apps can determine that they shouldn't even pick bad hosts
> in the first place.

Sounds reasonable.
Note there are a couple of other considerations when choosing the
physbits as reported to the guest:

  a) TCG's view - I think it had a fixed size of 40 bits, but I haven't
     dug into it.
  b) We recently gained 'host-phys-bits-limit' which when used with
host-phys-bits lets you take the host value but then limit it.  Eduardo
seems to have done that to limit the guest from flipping into 5-level
page tables.  Hmm I've not tried with chips that do 5-level - but maybe
we also need this if you expect to migrate to hosts that don't have it.

(I've also got a vague memory that there's a limit in some IOMMUs
address sizes, but I can't remember what the details were).

Dave


> 
> Regards,
> Daniel
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK