Re: [edk2-devel] GuestPhysAddrSize questions

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

On Thu, Feb 22, 2024 at 5:13 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
> Also, to clarify the hardware behavior, if hCR4.LA57=0 and host
> PhysAddrSize==52, then will guest physical addresses above 2^48
>
> 1) cause a reserved #PF in the guest, or
>
> 2) cause a non-present NPF exit in the hypervisor?
>
> I remember that several years ago we had a discussion on hCR4.LA57=0
> reducing the address space compared to MAXPHYADDR, but I cannot find the
> emails and also at the time I didn't notice GuestPhysAddrSize.

Found them! They say that "according to the design document, CPU will
report #NPF if the guest references a PA that is greater than 48 bits
while the hypervisor is in 4-level nested paging mode". That's nice,
because it's the same behavior as the affected Intel processors.

Paolo

> Anyhow, basically we would like GuestPhysAddrSize to be "redefined" as
>
>     Maximum usable physical address size in bits.  Physical addresses
>     above this size should not be used, but will not produce a "reserved"
>     page fault.  When this field is zero, all bits up to PhysAddrSize are
>     usable.  This field is expected to be nonzero only on guests where
>     the hypervisor is using nested paging.



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