Re: [PATCH Part2 RFC v4 37/40] KVM: SVM: Add support to handle the RMP nested page fault

[Sean Christopherson]

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.