linux-kernel - Re: [PATCH] kvm-pr: manage illegal instructions

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Message-ID: <3f99bce1-6fb7-b81d-26ca-24f1ca426c3e@redhat.com>
Date:	Tue, 17 May 2016 10:35:31 +0200
From:	Laurent Vivier <lvivier@...hat.com>
To:	Alexander Graf <agraf@...e.de>
Cc:	kvm@...r.kernel.org, kvm-ppc@...r.kernel.org,
	Michael Ellerman <mpe@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Gleb Natapov <gleb@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm-pr: manage illegal instructions



On 12/05/2016 16:23, Laurent Vivier wrote:
> 
> 
> On 12/05/2016 11:27, Alexander Graf wrote:
>> On 05/12/2016 11:10 AM, Laurent Vivier wrote:
>>>
>>> On 11/05/2016 13:49, Alexander Graf wrote:
>>>> On 05/11/2016 01:14 PM, Laurent Vivier wrote:
>>>>> On 11/05/2016 12:35, Alexander Graf wrote:
>>>>>> On 03/15/2016 09:18 PM, Laurent Vivier wrote:
>>>>>>> While writing some instruction tests for kvm-unit-tests for powerpc,
>>>>>>> I've found that illegal instructions are not managed correctly with
>>>>>>> kvm-pr,
>>>>>>> while it is fine with kvm-hv.
>>>>>>>
>>>>>>> When an illegal instruction (like ".long 0") is processed by kvm-pr,
>>>>>>> the kernel logs are filled with:
>>>>>>>
>>>>>>>         Couldn't emulate instruction 0x00000000 (op 0 xop 0)
>>>>>>>         kvmppc_handle_exit_pr: emulation at 700 failed (00000000)
>>>>>>>
>>>>>>> While the exception handler receives an interrupt for each
>>>>>>> instruction
>>>>>>> executed after the illegal instruction.
>>>>>>>
>>>>>>> Signed-off-by: Laurent Vivier <lvivier@...hat.com>
>>>>>>> ---
>>>>>>>     arch/powerpc/kvm/book3s_emulate.c | 4 +++-
>>>>>>>     1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/arch/powerpc/kvm/book3s_emulate.c
>>>>>>> b/arch/powerpc/kvm/book3s_emulate.c
>>>>>>> index 2afdb9c..4ee969d 100644
>>>>>>> --- a/arch/powerpc/kvm/book3s_emulate.c
>>>>>>> +++ b/arch/powerpc/kvm/book3s_emulate.c
>>>>>>> @@ -99,7 +99,6 @@ int kvmppc_core_emulate_op_pr(struct kvm_run *run,
>>>>>>> struct kvm_vcpu *vcpu,
>>>>>>>           switch (get_op(inst)) {
>>>>>>>         case 0:
>>>>>>> -        emulated = EMULATE_FAIL;
>>>>>>>             if ((kvmppc_get_msr(vcpu) & MSR_LE) &&
>>>>>>>                 (inst == swab32(inst_sc))) {
>>>>>>>                 /*
>>>>>>> @@ -112,6 +111,9 @@ int kvmppc_core_emulate_op_pr(struct kvm_run
>>>>>>> *run,
>>>>>>> struct kvm_vcpu *vcpu,
>>>>>>>                 kvmppc_set_gpr(vcpu, 3, EV_UNIMPLEMENTED);
>>>>>>>                 kvmppc_set_pc(vcpu, kvmppc_get_pc(vcpu) + 4);
>>>>>>>                 emulated = EMULATE_DONE;
>>>>>>> +        } else {
>>>>>>> +            kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
>>>>>> But isn't that exactly what the semantic of EMULATE_FAIL is? Fixing it
>>>>>> up in book3s_emulate.c is definitely the wrong spot.
>>>>>>
>>>>>> So what is the problem you're trying to solve? Is the SRR0 at the
>>>>>> wrong
>>>>>> spot or are the log messages the problem?
>>>>> No, the problem is the host kernel logs are filled by the message and
>>>>> the execution hangs. And the host becomes unresponsiveness, even after
>>>>> the end of the tests.
>>>>>
>>>>> Please, try to run kvm-unit-tests (the emulator test) on a KVM-PR host,
>>>>> and check the kernel logs (dmesg), then try to ssh to the host...
>>>> Ok, so the log messages are the problem. Please fix the message output
>>>> then - or remove it altogether. Or if you like, create a module
>>>> parameter that allows you to emit them.
>>>>
>>>> I personally think the best solution would be to just convert the
>>>> message into a trace point.
>>>>
>>>> While at it, please see whether the guest can trigger similar host log
>>>> output excess in other code paths.
>>> The problem is not really with the log messages: they are consequence of
>>> the bug I try to fix.
>>>
>>> What happens is once kvm_pr decodes an invalid instruction all the valid
>>> following instructions trigger a Program exception to the guest (but are
>>> executed correctly). It has no real consequence on big machine like
>>> POWER8, except that the guest become very slow and the log files of the
>>> host are filled with messages (and qemu uses 100% of the CPU). On a
>>> smaller machine like a  PowerMac G5, the machine becomes simply unusable.
>>
>> It's probably more related to your verbosity level of kernel messages.
>> If you pass loglevel=0 (or quiet) to you kernel cmdline you won't get
>> the messages printed to serial which is what's slowing you down.
>>
>> The other problem sounds pretty severe, but the only thing your patch
>> does any different from the current code flow would be the patch below.
>> Or did I miss anything?
>>
>> diff --git a/arch/powerpc/kvm/emulate.c b/arch/powerpc/kvm/emulate.c
>> index 5cc2e7a..4672bc2 100644
>> --- a/arch/powerpc/kvm/emulate.c
>> +++ b/arch/powerpc/kvm/emulate.c
>> @@ -302,7 +302,11 @@ int kvmppc_emulate_instruction(struct kvm_run *run,
>> struct kvm_vcpu *vcpu)
>>                         advance = 0;
>>                         printk(KERN_ERR "Couldn't emulate instruction
>> 0x%08x "
>>                                "(op %d xop %d)\n", inst, get_op(inst),
>> get_xop(inst));
>> +#ifdef CONFIG_PPC_BOOK3S
>> +                       kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
>> +#else
>>                         kvmppc_core_queue_program(vcpu, 0);
>> +#endif
>>                 }
>>         }
>>

Do you want I send an updated patch with your changes?

Laurent