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From: Thomas Garnier <thgarnie@google.com>
Date: Tue, 21 Mar 2017 13:25:24 -0700
Message-ID: <CAJcbSZFuJiJE0=rS-ZGPLCMUgaz+dPzHUm-iMa=J1E4H0qAHFg@mail.gmail.com>
Subject: Re: [lkp-robot] [x86] 69218e4799: BUG:kernel_hang_in_boot_stage
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: kernel test robot <xiaolong.ye@intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Borislav Petkov <bp@suse.de>, Chris Wilson <chris@chris-wilson.co.uk>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Jiri Kosina <jikos@kernel.org>, Joerg Roedel <joro@8bytes.org>,
        Jonathan Corbet <corbet@lwn.net>, Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <jgross@suse.com>, Kees Cook <keescook@chromium.org>,
        Len Brown <len.brown@intel.com>, Lorenzo Stoakes <lstoakes@gmail.com>,
        "Luis R . Rodriguez" <mcgrof@kernel.org>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        Michal Hocko <mhocko@suse.com>, Paolo Bonzini <pbonzini@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Pavel Machek <pavel@ucw.cz>, Peter Zijlstra <peterz@infradead.org>,
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        Thomas Gleixner <tglx@linutronix.de>,
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        Vitaly Kuznetsov <vkuznets@redhat.com>, zijun_hu <zijun_hu@htc.com>,
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The issue seems to be related to exceptions happening in close pages
to the fixmap GDT remapping.

The original page fault happen in do_test_wp_bit which set a fixmap
entry to test WP flag. If I grow the number of processors supported
increasing the distance between the remapped GDT page and the WP test
page, the error does not reproduce.

I am still looking at the exact distance between repro and no-repro as
well as the exact root cause.

On Tue, Mar 21, 2017 at 12:23 PM, Thomas Garnier <thgarnie@google.com> wrote:
> On Tue, Mar 21, 2017 at 12:20 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> On Tue, Mar 21, 2017 at 11:16 AM, Thomas Garnier <thgarnie@google.com> wrote:
>> > This error happens even with Andy TLS fix on 32-bit (GDT is on fixmap
>> > but not readonly). I am looking into it.
>> >
>> > KVM internal error. Suberror: 3
>> > extra data[0]: 80000b0e
>> > extra data[1]: 31
>>
>> If I read that right, it's extra data[1] 0x31, which EXIT_REASON_EPT_MISCONFIG.
>>
>> I'm not seeing how the A bit in a GDT entry could have anything to do
>> with it. I'm assuming it happens even without Andy's patch?
>
> Correct.
>
>>
>>                   Linus
>
>
>
>
> --
> Thomas


-- 
Thomas