Re: [edk2-devel] GuestPhysAddrSize questions

["Paolo Bonzini" <pbonzini@redhat.com>]

On Mon, Mar 4, 2024 at 6:24 PM Tom Lendacky <thomas.lendacky@amd.com> wrote:
>
> On 3/4/24 07:09, Gerd Hoffmann wrote:
> >    Hi,
> >
> >>>     23:16 GuestPhysAddrSize Maximum guest physical address size in bits.
> >>>                             This number applies only to guests using nested
> >>>                             paging. When this field is zero, refer to the
> >>>                             PhysAddrSize field for the maximum guest
> >>>                             physical address size. See “Secure Virtual
> >>>                             Machine” in APM Volume 2.
> >
> >> I believe the main purpose of GuestPhysAddrSize was for software use (for
> >> nested virtualization) and that the hardware itself has always returned zero
> >> for that value. So you should be able to use that field. Adding @Paolo for
> >> his thoughts.
> >
> > Reviewers mentioned this is meant for nested guests, i.e. (if I
> > understand this correctly) the l0 hypervisor can use that to tell
> > the l1 hypervisor what the l2 guest phys-bits should be.
> >
> > Is this nested virtualization use documented somewhere?  Tried to
> > search for GuestPhysAddrSize or Fn8000_0008_EAX in APM Volume 2,
> > found nothing.
>
> Right, and I don't think you'll see anything added to the APM that will
> state how it can be used by software. The APM is an architectural
> definition and won't talk about hypervisors and using nested paging, etc.

I don't think that's a problem. The problem is that the definition in
the APM is ambiguous and can mean one of three things:

1) it can be a suggested value for PhysAddrSize (bits 7:0) of guests
that use nested paging. This would imply that in a nested page guest,
with GuestPhysAddrSize=48, setting bits 51:48 would cause a
#PF(reserved) exception.

2) it can be equivalent to LinAddrSize (bits 15:8) but for nested page
tables. This would imply that, with GuestPhysAddrSize=48, VMRUN would
fail if hCR4.LA57=1.

3) it can be what I propose above: the architecture defined a
situation that can only happen when using nested paging (on AMD:
host_CR4.LA57=0, PhysAddrSize=52), and GuestPhysAddrSize is an
architectural way to explain the situation to guests.

The message above suggests that the intended meaning is (1). That is
because "the l0 hypervisor can use that to tell the l1 hypervisor what
the l2 guest phys-bits should be" is exactly the same as "the
processor can use that to tell the hypervisor what the guest phys-bits
should be" (just shifted one level down).

However, there are no processors that implement (1) or (2), so my
suggestion is to clarify that the intended meaning is (3). Do you
agree that the above proposal is a plausible interpretation of what is
already in the APM, but clearer? And do you think there is a way for
the clarification to make it into the APM?

Paolo



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