On Fri, Sep 7, 2012 at 10:49 AM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
> On 07/09/12 15:28, Will Deacon wrote:
>> On Thu, Sep 06, 2012 at 06:16:03PM +0100, Marc Zyngier wrote:
>>> Add minimal guest support to perf, so it can distinguish whether
>>> the PMU interrupt was in the host or the guest, as well as collecting
>>> some very basic information (guest PC, user vs kernel mode).
>>>
>>> This is not feature complete though, as it doesn't support backtracing
>>> in the guest.
>>>
>>> Based on the x86 implementation, tested with KVM/ARM.
>>>
>>> Cc: Will Deacon <will.deacon@xxxxxxx>
>>> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
>>> ---
>>> arch/arm/include/asm/perf_event.h | 7 ++++++-
>>> arch/arm/kernel/perf_event.c | 36 ++++++++++++++++++++++++++++++++++++
>>> 2 files changed, 42 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/arm/include/asm/perf_event.h b/arch/arm/include/asm/perf_event.h
>>> index e074948..77537bb 100644
>>> --- a/arch/arm/include/asm/perf_event.h
>>> +++ b/arch/arm/include/asm/perf_event.h
>>> @@ -12,6 +12,11 @@
>>> #ifndef __ARM_PERF_EVENT_H__
>>> #define __ARM_PERF_EVENT_H__
>>>
>>> -/* Nothing to see here... */
>>> +/* Not much to see here... */
>>
>> You can just remove this silly comment :)
>
> Now you're taking away the little fun I had hacking this. Life is soooo
> cruel... ;-)
>
>>> +struct pt_regs;
>>> +extern unsigned long perf_instruction_pointer(struct pt_regs *regs);
>>> +extern unsigned long perf_misc_flags(struct pt_regs *regs);
>>> +#define perf_misc_flags(regs) perf_misc_flags(regs)
>>>
>>> #endif /* __ARM_PERF_EVENT_H__ */
>>> diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
>>> index ab243b8..8836425 100644
>>> --- a/arch/arm/kernel/perf_event.c
>>> +++ b/arch/arm/kernel/perf_event.c
>>> @@ -817,6 +817,10 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
>>> {
>>> struct frame_tail __user *tail;
>>>
>>> + if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
>>> + /* TODO: We don't support guest os callchain now */
>>> + return;
>>> + }
>>>
>>> tail = (struct frame_tail __user *)regs->ARM_fp - 1;
>>>
>>> @@ -844,9 +848,41 @@ perf_callchain_kernel(struct perf_callchain_entry *entry, struct pt_regs *regs)
>>> {
>>> struct stackframe fr;
>>>
>>> + if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
>>> + /* TODO: We don't support guest os callchain now */
>>> + return;
>>> + }
>>> +
>>
>> Will we ever support this? It sounds incredibly difficult -- how can you
>> walk the guest stack without things disappearing under your feet?
>
> It's hard, but not impossible:
> - Stop the world (all vcpus)
> - Repeatedly call into HYP mode to resolve all the VAs needed by the
> backtrace (cringe...)
> - Restart the world
> - Witness your guest running at the speed of a snail lost in the Sahara.
>
> I reckon it is a perfect project for a summer student! :-)
>
>>> +unsigned long perf_instruction_pointer(struct pt_regs *regs)
>>> +{
>>> + if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
>>> + return perf_guest_cbs->get_guest_ip();
>>> +
>>> + return instruction_pointer(regs);
>>> +}
>>> +
>>> +unsigned long perf_misc_flags(struct pt_regs *regs)
>>> +{
>>> + int misc = 0;
>>> +
>>> + if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
>>> + if (perf_guest_cbs->is_user_mode())
>>> + misc |= PERF_RECORD_MISC_GUEST_USER;
>>> + else
>>> + misc |= PERF_RECORD_MISC_GUEST_KERNEL;
>>> + } else {
>>> + if (user_mode(regs))
>>> + misc |= PERF_RECORD_MISC_USER;
>>> + else
>>> + misc |= PERF_RECORD_MISC_KERNEL;
>>> + }
>>> +
>>> + return misc;
>>> +}
>>
>> Hmm, both of these look like they should be made generic but I see that x86
>> and ppc do have some differences, so I guess we should have our own copies
>> for the time being...
>
> We can probably factor in the common code and let ARM use the default.
> I'll have a look if I get a chance.
>
or, maybe we can just ask the guest nicely using some kind of PV api
and see if it does it for us...
-Christoffer
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