linux-kernel - Re: [FYI PATCH 1/3] KVM: nVMX: Don't emulate instructions in guest mode

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Date:   Sat, 29 Feb 2020 10:33:12 -0800
From:   Oliver Upton <oupton@...gle.com>
To:     Jan Kiszka <jan.kiszka@....de>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>, stable@...r.kernel.org,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [FYI PATCH 1/3] KVM: nVMX: Don't emulate instructions in guest mode

Hi Jan,

On Sat, Feb 29, 2020 at 10:00 AM Jan Kiszka <jan.kiszka@....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.

--
Thanks,
Oliver