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

[Paolo Bonzini <pbonzini@redhat.com>]

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