Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

[Andy Lutomirski <luto@kernel.org>]

On Sat, Mar 7, 2020 at 11:01 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Andy Lutomirski <luto@kernel.org> writes:
> > On Sat, Mar 7, 2020 at 7:47 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> >> The host knows exactly when it injects a async PF and it can store CR2
> >> and reason of that async PF in flight.
> >>
> >> On the next VMEXIT it checks whether apf_reason is 0. If apf_reason is 0
> >> then it knows that the guest has read CR2 and apf_reason. All good
> >> nothing to worry about.
> >>
> >> If not it needs to be careful.
> >>
> >> As long as the apf_reason of the last async #PF is not cleared by the
> >> guest no new async #PF can be injected. That's already correct because
> >> in that case IF==0 which prevents a nested async #PF.
> >>
> >> If MCE, NMI trigger a real pagefault then the #PF injection needs to
> >> clear apf_reason and set the correct CR2. When that #PF returns then the
> >> old CR2 and apf_reason need to be restored.
> >
> > How is the host supposed to know when the #PF returns?  Intercepting
> > IRET sounds like a bad idea and, in any case, is not actually a
> > reliable indication that #PF returned.
>
> The host does not care about the IRET. It solely has to check whether
> apf_reason is 0 or not. That way it knows that the guest has read CR2
> and apf_reason.

/me needs actual details

Suppose the host delivers an async #PF.  apf_reason != 0 and CR2
contains something meaningful.  Host resumes the guest.

The guest does whatever (gets NMI, and does perf stuff, for example).
The guest gets a normal #PF.  Somehow the host needs to do:

if (apf_reason != 0) {
  prev_apf_reason = apf_reason;
  prev_cr2 = cr2;
  apf_reason = 0;
  cr2 = actual fault address;
}

resume guest;

Obviously this can only happen if the host intercepts #PF.  Let's
pretend for now that this is even possible on SEV-ES (it may well be,
but I would also believe that it's not.  SEV-ES intercepts are weird
and I don't have the whole manual in my head.  I'm not sure the host
has any way to read CR2 for a SEV-ES guest.)  So now the guest runs
some more and finishes handling the inner #PF.  Some time between
doing that and running the outer #PF code that reads apf_reason, the
host needs to do:

apf_reason = prev_apf_reason;
cr2 = prev_cr2;
prev_apf_reason = 0;

How is the host supposed to know when to do that?

--Andy