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Date: Wed, 20 Apr 2016 21:05:07 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Andy Lutomirski <luto@...capital.net> Cc: Dmitry Safonov <dsafonov@...tuozzo.com>, Thomas Gleixner <tglx@...utronix.de>, Shuah Khan <shuahkh@....samsung.com>, Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, Borislav Petkov <bp@...en8.de>, khorenko@...tuozzo.com, X86 ML <x86@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, xemul@...tuozzo.com, linux-kselftest@...r.kernel.org, Cyrill Gorcunov <gorcunov@...nvz.org>, Dmitry Safonov <0x7f454c46@...il.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com> Subject: Re: [PATCH 1/2] x86/arch_prctl: add ARCH_SET_{COMPAT,NATIVE} to change compatible mode 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.
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