From: Andy Lutomirski <luto@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>,
LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
kvm list <kvm@vger.kernel.org>,
Paolo Bonzini <pbonzini@redhat.com>,
stable <stable@vger.kernel.org>
Subject: Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS
Date: Sat, 7 Mar 2020 11:34:57 -0800 [thread overview]
Message-ID: <CALCETrXGiZQG-h3nuXL4HZJyTJ4T2mjJhSvcqpVy8B9hr+qjNA@mail.gmail.com> (raw)
In-Reply-To: <87a74s9ehb.fsf@nanos.tec.linutronix.de>
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
next prev parent reply other threads:[~2020-03-07 19:35 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-07 2:26 [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS Andy Lutomirski
2020-03-07 15:03 ` Andy Lutomirski
2020-03-07 15:47 ` Thomas Gleixner
2020-03-07 15:59 ` Andy Lutomirski
2020-03-07 19:01 ` Thomas Gleixner
2020-03-07 19:34 ` Andy Lutomirski [this message]
2020-03-08 7:23 ` Thomas Gleixner
2020-03-09 6:57 ` Thomas Gleixner
2020-03-09 8:40 ` Paolo Bonzini
2020-03-09 9:09 ` Thomas Gleixner
2020-03-09 18:14 ` Andy Lutomirski
2020-03-09 19:05 ` Thomas Gleixner
2020-03-09 20:22 ` Peter Zijlstra
2020-04-06 19:09 ` Vivek Goyal
2020-04-06 20:25 ` Peter Zijlstra
2020-04-06 20:32 ` Andy Lutomirski
2020-04-06 20:42 ` Andy Lutomirski
2020-04-07 17:21 ` Vivek Goyal
2020-04-07 17:38 ` Andy Lutomirski
2020-04-07 20:20 ` Thomas Gleixner
2020-04-07 21:41 ` Andy Lutomirski
2020-04-07 22:07 ` Paolo Bonzini
2020-04-07 22:29 ` Andy Lutomirski
2020-04-08 0:30 ` Paolo Bonzini
2020-05-21 15:55 ` Vivek Goyal
2020-04-07 22:48 ` Thomas Gleixner
2020-04-08 4:48 ` Andy Lutomirski
2020-04-08 9:32 ` Borislav Petkov
2020-04-08 10:12 ` Thomas Gleixner
2020-04-08 18:23 ` Vivek Goyal
2020-04-07 22:49 ` Vivek Goyal
2020-04-08 10:01 ` Borislav Petkov
2020-04-07 22:04 ` Paolo Bonzini
2020-04-07 23:21 ` Thomas Gleixner
2020-04-08 8:23 ` Paolo Bonzini
2020-04-08 13:01 ` Thomas Gleixner
2020-04-08 15:38 ` Peter Zijlstra
2020-04-08 16:41 ` Thomas Gleixner
2020-04-09 9:03 ` Paolo Bonzini
2020-04-08 15:34 ` Sean Christopherson
2020-04-08 16:50 ` Paolo Bonzini
2020-04-08 18:01 ` Thomas Gleixner
2020-04-08 20:34 ` Vivek Goyal
2020-04-08 23:06 ` Thomas Gleixner
2020-04-08 23:14 ` Thomas Gleixner
2020-04-09 4:50 ` Andy Lutomirski
2020-04-09 9:43 ` Paolo Bonzini
2020-04-09 11:36 ` Andrew Cooper
2020-04-09 12:47 ` Paolo Bonzini
2020-04-09 14:13 ` Andrew Cooper
2020-04-09 14:32 ` Paolo Bonzini
2020-04-09 15:03 ` Andy Lutomirski
2020-04-09 15:17 ` Paolo Bonzini
2020-04-09 17:32 ` Andy Lutomirski
2020-04-06 21:32 ` Thomas Gleixner
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