Re: [edk2-devel] GuestPhysAddrSize questions

["Lendacky, Thomas via groups.io" <thomas.lendacky=amd.com@groups.io>]

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.
> 
> Posted patches for kernel
> https://lore.kernel.org/kvm/20240301101410.356007-1-kraxel@redhat.com/
> and qemu
> https://lore.kernel.org/kvm/20240301101713.356759-1-kraxel@redhat.com/
> (sorry forgot to Cc you).
> 
> 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.

> 
> Is there any case where the phys-bits limits for an l1 guest and
> l2 guest would be different?

I haven't really thought about this before and all the implications that 
may or may not be in play, so I don't think I can really answer that.

Thanks,
Tom

> 
> thanks & take care,
>    Gerd
> 


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