Re: [PATCH 2/2] x86/cpu: Handle GUEST_MAXPHYADDR < HOST_MAXPHYADDR for hosts that don't support it

Hi, Well, the fundamental issue is not that hard actually. We have three cases: (1) GUEST_MAXPHYADDR == HOST_MAXPHYADDR Everything is fine ;) (2) GUEST_MAXPHYADDR < HOST_MAXPHYADDR Mostly fine. Some edge cases, like different page fault errors for addresses above GUEST_MAXPHYADDR and below HOST_MAXPHYADDR. Which I think Mohammed fixed in the kernel recently. (3) GUEST_MAXPHYADDR > HOST_MAXPHYADDR Broken. If the guest uses addresses above HOST_MAXPHYADDR everything goes south. The (2) case isn't much of a problem. We only need to figure whenever we want qemu allow this unconditionally (current state) or only in case the kernel fixes are present (state with this patch applied if I read it correctly). The (3) case is the reason why guest firmware never ever uses GUEST_MAXPHYADDR and goes with very conservative heuristics instead, which in turn leads to the consequences discussed at length in the OVMF thread linked above. Ideally we would simply outlaw (3), but it's hard for backward compatibility reasons. Second best solution is a flag somewhere (msr, cpuid, ...) telling the guest firmware "you can use GUEST_MAXPHYADDR, we guarantee it is <= HOST_MAXPHYADDR". Yep, better defaults for GUEST_MAXPHYADDR would be good too, but that is a separate (although related) discussion. take care, Gerd