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

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

* Laszlo Ersek (lersek@redhat.com) wrote:
> On 06/16/20 19:14, Guilherme Piccoli wrote:
> > Thanks Gerd, Dave and Eduardo for the prompt responses!
> > 
> > So, I understand that when we use "-host-physical-bits", we are
> > passing the *real* number for the guest, correct? So, in this case we
> > can trust that the guest physbits matches the true host physbits.
> > 
> > What if then we have OVMF relying in the physbits *iff*
> > "-host-phys-bits" is used (which is the default in RH and a possible
> > machine configuration on libvirt XML in Ubuntu), and we have OVMF
> > fallbacks to 36-bit otherwise?
> 
> I've now read the commit message on QEMU commit 258fe08bd341d, and the
> complexity is simply stunning.
> 
> Right now, OVMF calculates the guest physical address space size from
> various range sizes (such as hotplug memory area end, default or
> user-configured PCI64 MMIO aperture), and derives the minimum suitable
> guest-phys address width from that address space size. This width is
> then exposed to the rest of the firmware with the CPU HOB (hand-off
> block), which in turn controls how the GCD (global coherency domain)
> memory space map is sized. Etc.
> 
> If QEMU can provide a *reliable* GPA width, in some info channel (CPUID
> or even fw_cfg), then the above calculation could be reversed in OVMF.
> We could take the width as a given (-> produce the CPU HOB directly),
> plus calculate the *remaining* address space between the GPA space size
> given by the width, and the end of the memory hotplug area end. If the
> "remaining size" were negative, then obviously QEMU would have been
> misconfigured, so we'd halt the boot. Otherwise, the remaining area
> could be used as PCI64 MMIO aperture (PEI memory footprint of DXE page
> tables be darned).
> 
> > Now, regarding the problem "to trust or not" in the guests' physbits,
> > I think it's an orthogonal discussion to some extent. It'd be nice to
> > have that check, and as Eduardo said, prevent migration in such cases.
> > But it's not really preventing OVMF big PCI64 aperture if we only
> > increase the aperture _when  "-host-physical-bits" is used_.
> 
> I don't know what exactly those flags do, but I doubt they are clearly
> visible to OVMF in any particular way.

The firmware should trust whatever it reads from the cpuid and thus gets
told from qemu; if qemu is doing the wrong thing there then that's our
problem and we need to fix it in qemu.

Dave

> Thanks
> Laszlo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK