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

[Gerd Hoffmann]

On Wed, Jun 17, 2020 at 10:16:55AM +0200, Christophe de Dinechin wrote:
> 
> 
> > Le 16 Jun 2020 à 18:50, Gerd Hoffmann <kraxel@redhat.com> a écrit :
> > 
> >  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.
> 
> Is the only reason unreliable guest physbits?

Yes.

> > 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.
> 
> What is the rationale for ever allowing guest physbits > host physbits?

I can't think of a good reason.  So probably simply historical reasons
and the fact that this isn't a problem with tcg.

take care,
  Gerd Hoffmann