Re: [PATCH 1/2] x86/arch_prctl: add ARCH_SET_{COMPAT,NATIVE} to change compatible mode

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Apr 20, 2016 at 12:05 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, Apr 20, 2016 at 08:40:23AM -0700, Andy Lutomirski wrote:
>> Do LBR, PEBS, and similar report user regs or do they merely want to
>> know the instruction format?  If the latter, I could whip up a tiny
>> function to do just that (like perf_get_regs_user but just for ABI --
>> it would be simpler).
>
> Just the instruction format, nothing else.
>
>> >> Peter, I got lost in the code that calls this.  Are regs coming from
>> >> the overflow interrupt's regs, current_pt_regs(), or
>> >> perf_get_regs_user?
>> >
>> > So get_perf_callchain() will get regs from:
>> >
>> >  - interrupt/NMI regs
>> >  - perf_arch_fetch_caller_regs()
>> >
>> > And when user && !user_mode(), we'll use:
>> >
>> >  - task_pt_regs() (which arguably should maybe be perf_get_regs_user())
>>
>> Could you point me to this bit of the code?
>
> kernel/events/callchain.c:198

But that only applies to the callchain code, right?  AFAICS the PEBS
code is invoked through the x86_pmu NMI handler and always gets the
IRQ regs.  Except for this case:

static inline void intel_pmu_drain_pebs_buffer(void)
{
    struct pt_regs regs;

    x86_pmu.drain_pebs(&regs);
}

which seems a bit confused.

I don't suppose we could arrange to pass something consistent into the
PEBS handlers...

Or is the PEBS code being called from the callchain code somehow?

I haven't dug in to the LBR code much.

>>
>> One call to perf_get_user_regs per interrupt shouldn't be too bad --
>> certainly much better then one per PEBS record.  One call to get user
>> ABI per overflow would be even less bad, but at that point, folding it
>> in to the PEBS code wouldn't be so bad either.
>
> Right; although note that the whole fixup_ip() thing requires a single
> record per interrupt (for we need the LBR state for each record in order
> to rewind).

So do earlier PEBS events not get rewound?  Or so we just program the
thing to only ever give us one event at a time?

>
> Also, HSW+ PEBS doesn't do the fixup anymore.
>
>> If I'm understanding this right (a big, big if), if we get a PEBS
>> overflow while running in user mode, we'll dump out the user regs (and
>> call perf_get_regs_user) and all the PEBS entries (subject to
>> exclude_kernel and with all the decoding magic).  So, in that case, we
>> call perf_get_user_regs.
>
> We only dump user regs if PERF_SAMPLE_REGS_USER, and in case we hit
> userspace userspace with the interrupt we use the interrupt regs; see
> perf_sample_regs_user().
>
>> If we get a PEBS overflow while running in kernel mode, we'll report
>> the kernel regs (if !exclude_kernel) and report the PEBS data as well.
>> If any of those records are in user mode, then, ideally, we'd invoke
>> perf_get_regs_user or similar *once* to get the ABI.  Although, if we
>> can get the user ABI efficiently enough, then maybe we don't care if
>> we call it once per PEBS record.
>
> Right, if we interrupt kernel mode, we'll call perf_get_regs_user() if
> PERF_SAMPLE_REGS_USER (| PERF_SAMPLE_STACK_USER).

But not in get_perf_callchain.  So we'll show the correct user *regs*
but not the current user callchain under some conditions, AFAICS.

>
> The problem here is that the overflow stuff is designed for a single
> 'event' per interrupt, so passing it data for multiple events is
> somewhat icky.

It also seems that there's a certain amount of confusion as to exactly
what "regs" means in various contexts.  Or at least I'm confused by
it.

--Andy