Re: [PATCH 0/2] expose host-phys-bits to guest

[Gerd Hoffmann]

On Fri, Sep 02, 2022 at 08:10:00AM +0800, Xiaoyao Li wrote:
> On 9/2/2022 12:17 AM, Gerd Hoffmann wrote:
> > On Thu, Sep 01, 2022 at 10:36:19PM +0800, Xiaoyao Li wrote:
> > > On 9/1/2022 9:58 PM, Gerd Hoffmann wrote:
> > > 
> > > > > Anyway, IMO, guest including guest firmware, should always consult 
> > > > > from
> > > > > CPUID leaf 0x80000008 for physical address length.
> > > > 
> > > > It simply can't for the reason outlined above.  Even if we fix qemu
> > > > today that doesn't solve the problem for the firmware because we want
> > > > backward compatibility with older qemu versions.  Thats why I want the
> > > > extra bit which essentially says "CPUID leaf 0x80000008 actually works".
> > > 
> > > I don't understand how it backward compatible with older qemu version. Old
> > > QEMU won't set the extra bit you introduced in this series, and all the
> > > guest created with old QEMU will become untrusted on CPUID leaf 
> > > 0x80000008 ?
> > 
> > Correct, on old qemu firmware will not trust CPUID leaf 0x80000008.
> > That is not worse than the situation we have today, currently the
> > firmware never trusts CPUID leaf 0x80000008.
> > 
> > So the patches will improves the situation for new qemu only, but I
> > don't see a way around that.
> > 
> 
> I see.
> 
> But IMHO, I don't think it's good that guest firmware workaround the issue
> on its own. Instead, it's better to just trust CPUID leaf 0x80000008 and
> fail if the given physical address length cannot be virtualized/supported.
> 
> It's just the bug of VMM to virtualize the physical address length. The
> correction direction is to fix the bug not the workaround to hide the bug.

I'm starting to repeat myself. "just trust CPUID leaf 0x80000008"
doesn't work because you simply can't with current qemu versions.

I don't like the dance with the new bit very much either, but I don't
see a better way without massive fallout due to compatibility problems.
I'm open to suggestions though.

take care,
  Gerd