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.
