Subject: Re: [PATCH] kvm-pr: manage illegal instructions

From: Alexander Graf <[email-masked]>
Date: Wed, 11 May 2016 13:49:19 +0200

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 <[email-masked]>
>>> ---
>>>    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.

Alex