Re: [PATCH 2/2] x86: KVM: Emulate instruction when GPA can't be translated by EPT

[Sean Christopherson <seanjc@google.com>]

On Mon, Dec 18, 2023, Tao Su wrote:
> With 4-level EPT, bits 51:48 of the guest physical address must all
> be zero; otherwise, an EPT violation always occurs, which is an unexpected
> VM exit in KVM currently.
> 
> Even though KVM advertises the max physical bits to guest, guest may
> ignore MAXPHYADDR in CPUID and set a bigger physical bits to KVM.
> Rejecting invalid guest physical bits on KVM side is a choice, but it will
> break current KVM ABI, e.g., current QEMU ignores the physical bits
> advertised by KVM and uses host physical bits as guest physical bits by
> default when using '-cpu host', although we would like to send a patch to
> QEMU, it will still cause backward compatibility issues.
> 
> For GPA that can't be translated by EPT but within host.MAXPHYADDR,
> emulation should be the best choice since KVM will inject #PF for the
> invalid GPA in guest's perspective and try to emulate the instructions
> which minimizes the impact on guests as much as possible.

NAK.  allow_smaller_maxphyaddr is a bit of a mess and in IMO was a mistake, but
at least there was reasonable motivation for trying to support guests with a small
MAXPHYADDR.  Fudging around a QEMU bug is not good enough justification, especially
since the odds of a hack in KVM fully working are slim to none.