linux-kernel - Re: [PATCH v1.1 3/3] x86/static_call: Add support for Jcc tail-calls

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Message-ID: <20230206110714.49fd7261@rorschach.local.home>
Date:   Mon, 6 Feb 2023 11:07:14 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        tglx@...utronix.de, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, jpoimboe@...nel.org,
        jbaron@...mai.com, ardb@...nel.org, linux-kernel@...r.kernel.org,
        erhard_f@...lbox.org, mhiramat@...nel.org, sandipan.das@....com
Subject: Re: [PATCH v1.1 3/3] x86/static_call: Add support for Jcc
 tail-calls

On Thu, 26 Jan 2023 10:14:49 -0800
Nick Desaulniers <ndesaulniers@...gle.com> wrote:

> On Thu, Jan 26, 2023 at 7:34 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Tue, Jan 24, 2023 at 10:07:53AM -0500, Steven Rostedt wrote:  
> > > On Tue, 24 Jan 2023 14:06:49 +0100
> > > Peter Zijlstra <peterz@...radead.org> wrote:
> > >  
> > > > > Just to confirm, as it's not clear if this is the static call site or one
> > > > > of the functions that is being called.  
> > > >
> > > > Ah, you've not looked at enough asm then? ;-) Yes this is the static
> > > > call site, see the __SCT_ target (instruction at 0x35d).  
> > >
> > > Yeah, could you specify it a bit more in the change log such that those
> > > looking back at this don't have to have that requirement of staring at
> > > enough asm ;-)  
> >
> > How's this then?

Better :-)

Anyway... Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

> >
> > ---
> > Subject: x86/static_call: Add support for Jcc tail-calls
> > From: Peter Zijlstra <peterz@...radead.org>
> > Date: Fri Jan 20 16:40:33 CET 2023
> >
> > Clang likes to create conditional tail calls like:
> >
> > 0000000000000350 <amd_pmu_add_event>:
> > 350:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1) 351: R_X86_64_NONE      __fentry__-0x4
> > 355:       48 83 bf 20 01 00 00 00         cmpq   $0x0,0x120(%rdi)
> > 35d:       0f 85 00 00 00 00       jne    363 <amd_pmu_add_event+0x13>     35f: R_X86_64_PLT32     __SCT__amd_pmu_branch_add-0x4
> > 363:       e9 00 00 00 00          jmp    368 <amd_pmu_add_event+0x18>     364: R_X86_64_PLT32     __x86_return_thunk-0x4  
> 
> If it helps reduce the line width in the commit message to focus on
> just the instructions, lately I've been using
> `--no-addresses --no-show-raw-insn` flags to llvm-objdump.

Actually, I like the full output. Even if we do reduce it, it will
still be over 80.

-- Steve