Re: [PATCH 2/2] KVM: PPC: Book3E: Emulate MCSRR0/1 SPR and rfmci instruction

[Scott Wood <scottwood@freescale.com>]

On 07/09/2013 12:46:32 PM, Alexander Graf wrote:
> On 07/09/2013 07:16 PM, Scott Wood wrote:
>> On 07/08/2013 01:45:58 PM, Alexander Graf wrote:
>>> 
>>> On 03.07.2013, at 15:30, Mihai Caraman wrote:
>>> 
>>> > Some guests are making use of return from machine check  
>>> instruction
>>> > to do crazy things even though the 64-bit kernel doesn't handle  
>>> yet
>>> > this interrupt. Emulate MCSRR0/1 SPR and rfmci instruction  
>>> accordingly.
>>> >
>>> > Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
>>> > ---
>>> > arch/powerpc/include/asm/kvm_host.h |    1 +
>>> > arch/powerpc/kvm/booke_emulate.c    |   25  
>>> +++++++++++++++++++++++++
>>> > arch/powerpc/kvm/timing.c           |    1 +
>>> > 3 files changed, 27 insertions(+), 0 deletions(-)
>>> >
>>> > diff --git a/arch/powerpc/include/asm/kvm_host.h  
>>> b/arch/powerpc/include/asm/kvm_host.h
>>> > index af326cd..0466789 100644
>>> > --- a/arch/powerpc/include/asm/kvm_host.h
>>> > +++ b/arch/powerpc/include/asm/kvm_host.h
>>> > @@ -148,6 +148,7 @@ enum kvm_exit_types {
>>> >     EMULATED_TLBWE_EXITS,
>>> >     EMULATED_RFI_EXITS,
>>> >     EMULATED_RFCI_EXITS,
>>> > +    EMULATED_RFMCI_EXITS,
>>> 
>>> I would quite frankly prefer to see us abandon the whole exit  
>>> timing framework in the kernel and instead use trace points. Then  
>>> we don't have to maintain all of this randomly exercised code.
>> 
>> Would this map well to tracepoints?  We're not trying to track  
>> discrete events, so much as accumulated time spent in different  
>> areas.
> 
> I think so. We'd just have to emit tracepoints as soon as we enter  
> handle_exit and in prepare_to_enter. Then a user space program should  
> have everything it needs to create statistics out of that. It would  
> certainly simplify the entry/exit path.

I was hoping that wasn't going to be your answer. :-)

Such a change would introduce a new dependency, more complexity, and  
the possibility for bad totals to result from a ring buffer filling  
faster than userspace can drain it.

I also don't see how it would simplify entry/exit, since we'd still  
need to take timestamps in the same places, in order to record a final  
event that says how long a particular event took.

-Scott