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Subject: Re: [FYI PATCH 1/3] KVM: nVMX: Don't emulate instructions in guest
 mode
To:     Jim Mattson <jmattson@google.com>
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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From:   Jan Kiszka <jan.kiszka@web.de>
Message-ID: <3b63f015-8a00-9aae-87a0-8713195001aa@web.de>
Date:   Sat, 29 Feb 2020 21:36:56 +0100
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On 29.02.20 21:17, Jim Mattson wrote:
> Since UMIP emulation is broken, I'm not sure why anyone would use it.
> (Sorry, Paolo.)

FWIW, adding "-umip" to the "-cpu host" in my qemu setup works around
the bug.

Jan

>
> 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> wrot=
e:
>>>>
>>>> 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-L=
inux
>>>>> 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', s=
o
>>>> 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 exitin=
g
>>>> 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