linux-kernel - Re: [RFC PATCH v2] perf/x86: make perf callchain work without CONFIG_FRAME

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Date:   Tue, 16 Apr 2019 19:45:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kairui Song <kasong@...hat.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Dave Young <dyoung@...hat.com>
Subject: Re: [RFC PATCH v2] perf/x86: make perf callchain work without
 CONFIG_FRAME_POINTER

On Wed, Apr 17, 2019 at 01:39:19AM +0800, Kairui Song wrote:
> And I also think the "fake"/"real" reg is fragile, could we abuse
> another eflag (just like PERF_EFLAGS_EXACT) to indicate the regs are
> partially dumped fake registers? 

Sure, the SDM seems to suggest bits 1,3,5,15 are 'available'. We've
already used 3 and 5, and I think we can use !X86_EFLAGS_FIXED to
indicate a fake regs set. Any real regs set will always have that set.