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

[Gerd Hoffmann]

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.

take care,
  Gerd