Message-ID: <53AFE2B3.5080300@web.de>
Date: Sun, 29 Jun 2014 11:56:03 +0200
From: Jan Kiszka <jan.kiszka@web.de>
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To: Gleb Natapov <gleb@kernel.org>, Borislav Petkov <bp@alien8.de>
CC: Paolo Bonzini <pbonzini@redhat.com>, lkml <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>, x86-ml <x86@kernel.org>,
        kvm@vger.kernel.org,
        =?ISO-8859-1?Q?J=F6rg_R=F6?= =?ISO-8859-1?Q?del?= 
	<joro@8bytes.org>
Subject: Re: __schedule #DF splat
References: <20140625153227.GA13845@pd.tnic> <20140625202650.GC13845@pd.tnic> <20140627101831.GB23153@pd.tnic> <53AD586A.40900@redhat.com> <20140627115545.GC23153@pd.tnic> <53AD5D27.2090505@redhat.com> <20140627121053.GD23153@pd.tnic> <20140628114431.GB4373@pd.tnic> <20140629064626.GD18167@minantech.com>
In-Reply-To: <20140629064626.GD18167@minantech.com>
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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: addres=
s 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 ex=
cp rip 0xffffffff8161130f info 2 7fffb62ba318
>>  qemu-system-x86-20240 [006] ...1  9406.484138: kvm_page_fault: addres=
s 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.
>=20
>> Or something else I'm missing, most probably...
>>
> Strange indeed. Can you also enable kvmmmu tracing? You can also instru=
ment
> kvm_multiple_exception() to see which two exception are combined into #=
DF.
>=20

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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