linux-kernel - Re: __schedule #DF splat

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Date:	Sun, 29 Jun 2014 11:56:03 +0200
From:	Jan Kiszka <jan.kiszka@....de>
To:	Gleb Natapov <gleb@...nel.org>, Borislav Petkov <bp@...en8.de>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>, x86-ml <x86@...nel.org>,
	kvm@...r.kernel.org,
	Jörg Rödel 
	<joro@...tes.org>
Subject: Re: __schedule #DF splat

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.

Jan



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