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Subject: Re: KCSAN + KVM = host reset
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <CANpmjNNUn9_Q30CSeqbU_TNvaYrMqwXkKCA23xO4ZLr2zO0w9Q@mail.gmail.com>
Date:   Thu, 9 Apr 2020 17:28:45 -0400
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        "paul E. McKenney" <paulmck@kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm@vger.kernel.org
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To:     Marco Elver <elver@google.com>
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> On Apr 9, 2020, at 12:03 PM, Marco Elver <elver@google.com> wrote:
>=20
> On Thu, 9 Apr 2020 at 17:30, Qian Cai <cai@lca.pw> wrote:
>>=20
>>=20
>>=20
>>> On Apr 9, 2020, at 11:22 AM, Marco Elver <elver@google.com> wrote:
>>>=20
>>> On Thu, 9 Apr 2020 at 17:10, Qian Cai <cai@lca.pw> wrote:
>>>>=20
>>>>=20
>>>>=20
>>>>> On Apr 9, 2020, at 3:03 AM, Marco Elver <elver@google.com> wrote:
>>>>>=20
>>>>> On Wed, 8 Apr 2020 at 23:29, Qian Cai <cai@lca.pw> wrote:
>>>>>>=20
>>>>>>=20
>>>>>>=20
>>>>>>> On Apr 8, 2020, at 5:25 PM, Paolo Bonzini <pbonzini@redhat.com> =
wrote:
>>>>>>>=20
>>>>>>> On 08/04/20 22:59, Qian Cai wrote:
>>>>>>>> Running a simple thing on this AMD host would trigger a reset =
right away.
>>>>>>>> Unselect KCSAN kconfig makes everything work fine (the host =
would also
>>>>>>>> reset If only "echo off > /sys/kernel/debug/kcsan=E2=80=9D =
before running qemu-kvm).
>>>>>>>=20
>>>>>>> Is this a regression or something you've just started to play =
with?  (If
>>>>>>> anything, the assembly language conversion of the AMD world =
switch that
>>>>>>> is in linux-next could have reduced the likelihood of such a =
failure,
>>>>>>> not increased it).
>>>>>>=20
>>>>>> I don=E2=80=99t remember I had tried this combination before, so =
don=E2=80=99t know if it is a
>>>>>> regression or not.
>>>>>=20
>>>>> What happens with KASAN? My guess is that, since it also happens =
with
>>>>> "off", something that should not be instrumented is being
>>>>> instrumented.
>>>>=20
>>>> No, KASAN + KVM works fine.
>>>>=20
>>>>>=20
>>>>> What happens if you put a 'KCSAN_SANITIZE :=3D n' into
>>>>> arch/x86/kvm/Makefile? Since it's hard for me to reproduce on this
>>>>=20
>>>> Yes, that works, but this below alone does not work,
>>>>=20
>>>> KCSAN_SANITIZE_kvm-amd.o :=3D n
>>>=20
>>> There are some other files as well, that you could try until you hit
>>> the right one.
>>>=20
>>> But since this is in arch, 'KCSAN_SANITIZE :=3D n' wouldn't be too =
bad
>>> for now. If you can't narrow it down further, do you want to send a
>>> patch?
>>=20
>> No, that would be pretty bad because it will disable KCSAN for Intel
>> KVM as well which is working perfectly fine right now. It is only AMD
>> is broken.
>=20
> Interesting. Unfortunately I don't have access to an AMD machine right =
now.
>=20
> Actually I think it should be:
>=20
>  KCSAN_SANITIZE_svm.o :=3D n
>  KCSAN_SANITIZE_pmu_amd.o :=3D n
>=20
> If you want to disable KCSAN for kvm-amd.

KCSAN_SANITIZE_svm.o :=3D n

That alone works fine. I am wondering which functions there could =
trigger
perhaps some kind of recursing with KCSAN?=