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From:   Jim Mattson <jmattson@google.com>
Date:   Sat, 29 Feb 2020 12:17:37 -0800
Message-ID: <CALMp9eR9zcXO64n7hDTzZHPkfV8qtuMHAbc=nLX8S8x7Crum+A@mail.gmail.com>
Subject: Re: [FYI PATCH 1/3] KVM: nVMX: Don't emulate instructions in guest mode
To:     Jan Kiszka <jan.kiszka@web.de>
Cc:     Oliver Upton <oupton@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>, stable@vger.kernel.org
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Since UMIP emulation is broken, I'm not sure why anyone would use it.
(Sorry, Paolo.)

On Sat, Feb 29, 2020 at 11:21 AM Jan Kiszka <jan.kiszka@web.de> wrote:
>
> On 29.02.20 20:00, Jim Mattson wrote:
> > On Sat, Feb 29, 2020 at 10:33 AM Oliver Upton <oupton@google.com> wrote:
> >>
> >> Hi Jan,
> >>
> >> On Sat, Feb 29, 2020 at 10:00 AM Jan Kiszka <jan.kiszka@web.de> wrote:
> >>> Is this expected to cause regressions on less common workloads?
> >>> Jailhouse as L1 now fails when Linux as L2 tries to boot a CPU: L2-Linux
> >>> gets a triple fault on load_current_idt() in start_secondary(). Only
> >>> bisected so far, didn't debug further.
> >>
> >> I'm guessing that Jailhouse doesn't use 'descriptor table exiting', so
> >> when KVM gets the corresponding exit from L2 the emulation burden is
> >> on L0. We now refuse the emulation, which kicks a #UD back to L2. I
> >> can get a patch out quickly to address this case (like the PIO exiting
> >> one that came in this series) but the eventual solution is to map
> >> emulator intercept checks into VM-exits + call into the
> >> nested_vmx_exit_reflected() plumbing.
> >
> > If Jailhouse doesn't use descriptor table exiting, why is L0
> > intercepting descriptor table instructions? Is this just so that L0
> > can partially emulate UMIP on hardware that doesn't support it?
> >
>
> That seems to be the case: My host lacks umip, L1 has it. So, KVM is
> intercepting descriptor table load instructions to emulate umip.
> Jailhouse never activates that interception.
>
> Jan