On Tue, Jul 20, 2021, Brijesh Singh wrote: > > On 7/20/21 5:31 PM, Sean Christopherson wrote: > ... > >> This is a good question, the GHCB spec does not enforce that a guest *must* > >> use page state. If the page state changes is not done by the guest then it > >> will cause #NPF and its up to the hypervisor to decide on what it wants to > >> do. > > Drat. Is there any hope of pushing through a GHCB change to require the guest > > to use PSC? > > Well, I am not sure if we can push it through GHCB. Other hypervisor > also need to agree to it. We need to define them some architectural way > for hypervisor to detect the violation and notify guest about it. And other guest's, too :-/ > >>> It would simplify KVM (albeit not much of a simplificiation) and would also > >>> make debugging easier since transitions would require an explicit guest > >>> request and guest bugs would result in errors instead of random > >>> corruption/weirdness. > >> I am good with enforcing this from the KVM. But the question is, what fault > >> we should inject in the guest when KVM detects that guest has issued the > >> page state change. > > Injecting a fault, at least from KVM, isn't an option since there's no architectural > > behavior we can leverage. E.g. a guest that isn't enlightened enough to properly > > use PSC isn't going to do anything useful with a #MC or #VC. > > > > Sadly, as is I think our only options are to either automatically convert RMP > > entries as need, or to punt the exit to userspace. Maybe we could do both, e.g. > > have a module param to control the behavior? The problem with punting to userspace > > is that KVM would also need a way for userspace to fix the issue, otherwise we're > > just taking longer to kill the guest :-/ > > > I think we should automatically convert the RMP entries at time, its > possible that non Linux guest may access the page without going through > the PSC. Agreed. I don't love that KVM will disallow automatic conversions when the host is accessing guest memory, but not when the guest is accessing memory. On the other hand, auto-converting when accessing from the host is far, far worse. And FWIW, IIRC this is also aligns with the expected/proposed TDX behavior, so that's a plus.
