linux-kernel - Re: KCSAN + KVM = host reset

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Date:   Thu, 9 Apr 2020 17:28:45 -0400
From:   Qian Cai <cai@....pw>
To:     Marco Elver <elver@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "paul E. McKenney" <paulmck@...nel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org
Subject: Re: KCSAN + KVM = host reset



> On Apr 9, 2020, at 12:03 PM, Marco Elver <elver@...gle.com> wrote:
> 
> On Thu, 9 Apr 2020 at 17:30, Qian Cai <cai@....pw> wrote:
>> 
>> 
>> 
>>> On Apr 9, 2020, at 11:22 AM, Marco Elver <elver@...gle.com> wrote:
>>> 
>>> On Thu, 9 Apr 2020 at 17:10, Qian Cai <cai@....pw> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Apr 9, 2020, at 3:03 AM, Marco Elver <elver@...gle.com> wrote:
>>>>> 
>>>>> On Wed, 8 Apr 2020 at 23:29, Qian Cai <cai@....pw> wrote:
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>>> On Apr 8, 2020, at 5:25 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>>>>>> 
>>>>>>> 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” before running qemu-kvm).
>>>>>>> 
>>>>>>> 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).
>>>>>> 
>>>>>> I don’t remember I had tried this combination before, so don’t know if it is a
>>>>>> regression or not.
>>>>> 
>>>>> What happens with KASAN? My guess is that, since it also happens with
>>>>> "off", something that should not be instrumented is being
>>>>> instrumented.
>>>> 
>>>> No, KASAN + KVM works fine.
>>>> 
>>>>> 
>>>>> What happens if you put a 'KCSAN_SANITIZE := n' into
>>>>> arch/x86/kvm/Makefile? Since it's hard for me to reproduce on this
>>>> 
>>>> Yes, that works, but this below alone does not work,
>>>> 
>>>> KCSAN_SANITIZE_kvm-amd.o := n
>>> 
>>> There are some other files as well, that you could try until you hit
>>> the right one.
>>> 
>>> But since this is in arch, 'KCSAN_SANITIZE := n' wouldn't be too bad
>>> for now. If you can't narrow it down further, do you want to send a
>>> patch?
>> 
>> 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.
> 
> Interesting. Unfortunately I don't have access to an AMD machine right now.
> 
> Actually I think it should be:
> 
>  KCSAN_SANITIZE_svm.o := n
>  KCSAN_SANITIZE_pmu_amd.o := n
> 
> If you want to disable KCSAN for kvm-amd.

KCSAN_SANITIZE_svm.o := n

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