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 7/19/21 7:10 PM, Sean Christopherson wrote:
> > On Wed, Jul 07, 2021, Brijesh Singh wrote:
> > > Follow the recommendation from APM2 section 15.36.10 and 15.36.11 to
> > > resolve the RMP violation encountered during the NPT table walk.
> > 
> > Heh, please elaborate on exactly what that recommendation is.  A recommendation
> > isn't exactly architectural, i.e. is subject to change :-)
> 
> I will try to expand it :)
> 
> > 
> > And, do we have to follow the APM's recommendation?
> 
> Yes, unless we want to be very strict on what a guest can do.
> 
> > Specifically, can KVM treat #NPF RMP violations as guest errors, or is that
> > not allowed by the GHCB spec?
> 
> The GHCB spec does not say anything about the #NPF RMP violation error. And
> not all #NPF RMP is a guest error (mainly those size mismatch etc).
> 
> > I.e. can we mandate accesses be preceded by page state change requests?
> 
> 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?

> > 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 :-/