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

[Peter Zijlstra]

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

> > to call perf_callchain_user(), which then, ands up calling
> > perf_callchain_user32() which is expected to NO-OP for 64bit userspace.
> >
> >> If it's the perf_get_regs_user, then this should be okay, but passing
> >> in the ABI field directly would be even nicer.  If they're coming from
> >> the overflow interrupt's regs or current_pt_regs(), could we change
> >> that?
> >>
> >> It might also be nice to make sure that we call perf_get_regs_user
> >> exactly once per overflow interrupt -- i.e. we could push it into the
> >> main code rather than the regs sampling code.
> >
> > The risk there is that we might not need the user regs at all to handle
> > the overflow thingy, so doing it unconditionally would be unwanted.
> 
> 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).

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).

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.