Re: [edk2-devel] GuestPhysAddrSize questions

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

On 2/22/24 05:24, Gerd Hoffmann wrote:
>    Hi,
> 
>> +    if (Cr4.Bits.LA57) {
>> +      if (PhysBits > 48) {
>> +        /*
>> +         * Some Intel CPUs support 5-level paging, have more than 48
>> +         * phys-bits but support only 4-level EPT, which effectively
>> +         * limits guest phys-bits to 48.
>> +         *
>> +         * AMD Processors have a different but somewhat related
>> +         * problem: They can handle guest phys-bits larger than 48
>> +         * only in case the host runs in 5-level paging mode.
>> +         *
>> +         * Until we have some way to communicate that kind of
>> +         * limitations from hypervisor to guest, limit phys-bits
>> +         * to 48 unconditionally.
>> +         */
> 
> So I'm looking for some communication path.  One option would be to use
> some bits in the KVM cpuid leaves.  Another possible candidate is cpuid
> leaf 0x80000008.
> 
>  From the AMD APM (revision 3.35):
> 
>    CPUID Fn8000_0008_EAX Long Mode Size Identifiers
>    ------------------------------------------------
> 
>    The value returned in EAX provides information about the maximum host
>    and guest physical and linear address width (in bits) supported by the
>    processor.
> 
>    Bits   FieldName        Description
> 
>    31:24  —                Reserved
> 
>    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.
> 
>    15:8  LinAddrSize       Maximum linear address size in bits.
> 
>    7:0   PhysAddrSize      Maximum physical address size in bits. When
>                            GuestPhysAddrSize is zero, this field also
>                            indicates the maximum guest physical address
>                            size.
> 
> The description of the GuestPhysAddrSize is somewhat vague.  Is this a
> value the hypervisor should use to figure how much address space it can
> give to guests?  Or is this a value the hypervisor can set to inform the
> guest about the available address space (which would be a solution to
> the problem outlined in the comment above)?  Or both?

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.

Thanks,
Tom

> 
> In case GuestPhysAddrSize should not be used this way:  Is it possible
> allocate the currently reserved bits 31:24 for that purpose?
> 
> Tom?  Michael?
> 
> thanks & take care,
>    Gerd
> 


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