linux-kernel - Re: [RFC KVM 06/27] KVM: x86: Exit KVM isolation on IRQ entry

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Date:   Mon, 13 May 2019 11:13:34 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        kvm list <kvm@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        jan.setjeeilers@...cle.com, Liran Alon <liran.alon@...cle.com>,
        Jonathan Adams <jwadams@...gle.com>
Subject: Re: [RFC KVM 06/27] KVM: x86: Exit KVM isolation on IRQ entry

On Mon, May 13, 2019 at 9:28 AM Alexandre Chartre
<alexandre.chartre@...cle.com> wrote:
>
>
>
> On 5/13/19 5:51 PM, Andy Lutomirski wrote:
> > On Mon, May 13, 2019 at 7:39 AM Alexandre Chartre
> > <alexandre.chartre@...cle.com> wrote:
> >>
> >> From: Liran Alon <liran.alon@...cle.com>
> >>
> >> Next commits will change most of KVM #VMExit handlers to run
> >> in KVM isolated address space. Any interrupt handler raised
> >> during execution in KVM address space needs to switch back
> >> to host address space.
> >>
> >> This patch makes sure that IRQ handlers will run in full
> >> host address space instead of KVM isolated address space.
> >
> > IMO this needs to be somewhere a lot more central.  What about NMI and
> > MCE?  Or async page faults?  Or any other entry?
> >
>
> Actually, I am not sure this is effectively useful because the IRQ
> handler is probably faulting before it tries to exit isolation, so
> the isolation exit will be done by the kvm page fault handler. I need
> to check that.
>

The whole idea of having #PF exit with a different CR3 than was loaded
on entry seems questionable to me.  I'd be a lot more comfortable with
the whole idea if a page fault due to accessing the wrong data was an
OOPS and the code instead just did the right thing directly.

--Andy