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

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Message-ID: <374d3217-cf6c-fa2e-d24a-e9ee8b1f4534@redhat.com>
Date:	Thu, 12 May 2016 16:23:13 +0200
From:	Laurent Vivier <lvivier@...hat.com>
To:	Alexander Graf <agraf@...e.de>, kvm@...r.kernel.org,
	kvm-ppc@...r.kernel.org
Cc:	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 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
>                 }
>         }
> 

OK, that fixes the problem. Thanks. :)

Could you apply it?

Laurent