Re: __schedule #DF splat

[Jan Kiszka]

On 2014-06-29 12:53, Gleb Natapov wrote:
> On Sun, Jun 29, 2014 at 12:31:50PM +0200, Jan Kiszka wrote:
>> On 2014-06-29 12:24, Gleb Natapov wrote:
>>> On Sun, Jun 29, 2014 at 11:56:03AM +0200, Jan Kiszka wrote:
>>>> On 2014-06-29 08:46, Gleb Natapov wrote:
>>>>> On Sat, Jun 28, 2014 at 01:44:31PM +0200, Borislav Petkov wrote:
>>>>>>  qemu-system-x86-20240 [006] ...1  9406.484134: kvm_page_fault: address 7fffb62ba318 error_code 2
>>>>>>  qemu-system-x86-20240 [006] ...1  9406.484136: kvm_inj_exception: #PF (0x2)a
>>>>>>
>>>>>> kvm injects the #PF into the guest.
>>>>>>
>>>>>>  qemu-system-x86-20240 [006] d..2  9406.484136: kvm_entry: vcpu 1
>>>>>>  qemu-system-x86-20240 [006] d..2  9406.484137: kvm_exit: reason PF excp rip 0xffffffff8161130f info 2 7fffb62ba318
>>>>>>  qemu-system-x86-20240 [006] ...1  9406.484138: kvm_page_fault: address 7fffb62ba318 error_code 2
>>>>>>  qemu-system-x86-20240 [006] ...1  9406.484141: kvm_inj_exception: #DF (0x0)
>>>>>>
>>>>>> Second #PF at the same address and kvm injects the #DF.
>>>>>>
>>>>>> BUT(!), why?
>>>>>>
>>>>>> I probably am missing something but WTH are we pagefaulting at a
>>>>>> user address in context_switch() while doing a lockdep call, i.e.
>>>>>> spin_release? We're not touching any userspace gunk there AFAICT.
>>>>>>
>>>>>> Is this an async pagefault or so which kvm is doing so that the guest
>>>>>> rip is actually pointing at the wrong place?
>>>>>>
>>>>> There is nothing in the trace that point to async pagefault as far as I see.
>>>>>
>>>>>> Or something else I'm missing, most probably...
>>>>>>
>>>>> Strange indeed. Can you also enable kvmmmu tracing? You can also instrument
>>>>> kvm_multiple_exception() to see which two exception are combined into #DF.
>>>>>
>>>>
>>>> FWIW, I'm seeing the same issue here (likely) on an E-450 APU. It
>>>> disappears with older KVM (didn't bisect yet, some 3.11 is fine) and
>>>> when patch-disabling the vmport in QEMU.
>>>>
>>>> Let me know if I can help with the analysis.
>>>>
>>> Bisection would be great of course. Once thing that is special about
>>> vmport that comes to mind is that it reads vcpu registers to userspace and
>>> write them back. IIRC "info registers" does the same. Can you see if the
>>> problem is reproducible with disabled vmport, but doing "info registers"
>>> in qemu console? Although trace does not should any exists to userspace
>>> near the failure...
>>
>> Yes, info registers crashes the guest after a while as well (with
>> different backtrace due to different context).
>>
> Oh crap. Bisection would be most helpful. Just to be absolutely sure
> that this is not QEMU problem: does exactly same QEMU version work with
> older kernels?

Yes, that was the case last time I tried (I'm on today's git head with
QEMU right now).

Will see what I can do regarding bisecting. That host is a bit slow
(netbook), so it may take a while. Boris will probably beat me in this.

Jan

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