Re: [PATCH 1/1] kvm: add support for guest physical bits

[Paolo Bonzini]

On 3/4/24 02:54, Xiaoyao Li wrote:
> On 3/1/2024 6:17 PM, Gerd Hoffmann wrote:
> > query kvm for supported guest physical address bits using
> > KVM_CAP_VM_GPA_BITS.  Expose the value to the guest via cpuid
> > (leaf 0x80000008, eax, bits 16-23).
> >
> > Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> > ---
> >   target/i386/cpu.h     | 1 +
> >   target/i386/cpu.c     | 1 +
> >   target/i386/kvm/kvm.c | 8 ++++++++
> >   3 files changed, 10 insertions(+)
> >
> > diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> > index 952174bb6f52..d427218827f6 100644
> > --- a/target/i386/cpu.h
> > +++ b/target/i386/cpu.h
> > @@ -2026,6 +2026,7 @@ struct ArchCPU {
> >       /* Number of physical address bits supported */
> >       uint32_t phys_bits;
> > +    uint32_t guest_phys_bits;
> >       /* in order to simplify APIC support, we leave this pointer to the
> >          user */
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > index 2666ef380891..1a6cfc75951e 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -6570,6 +6570,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t 
> > index, uint32_t count,
> >           if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
> >               /* 64 bit processor */
> >                *eax |= (cpu_x86_virtual_addr_width(env) << 8);
> > +             *eax |= (cpu->guest_phys_bits << 16);
>
> I think you misunderstand this field.
>
> If you expose this field to guest, it's the information for nested 
> guest. i.e., the guest itself runs as a hypervisor will know its nested 
> guest can have guest_phys_bits for physical addr.

It's one possible interpretation of AMD's definition. However there's no 
processor that has different MAXPHYADDR with/without nested paging, so 
there's no real benefit in adopting that interpretation.

The only architectural case in which you have two conflicting values for 
the guest MAXPHYADDR is hCR4.LA57=0 (and likewise for Intel 4-level EPT) 
with MAXPHYADDR=52, so it's useful to treat GuestPhysAddrSize as a way 
to communicate this situation to the guest.

Paolo