From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: mingo@kernel.org, tglx@linutronix.de, bp@alien8.de,
dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com,
jpoimboe@kernel.org, jbaron@akamai.com, ardb@kernel.org,
linux-kernel@vger.kernel.org, erhard_f@mailbox.org,
ndesaulniers@google.com, mhiramat@kernel.org,
sandipan.das@amd.com
Subject: Re: [PATCH 3/3] x86/static_call: Add support for Jcc tail-calls
Date: Tue, 24 Jan 2023 14:06:49 +0100 [thread overview]
Message-ID: <Y8/X6fdRT80jssIf@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20230123174431.4421dfdf@gandalf.local.home>
On Mon, Jan 23, 2023 at 05:44:31PM -0500, Steven Rostedt wrote:
> On Mon, 23 Jan 2023 21:59:18 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
>
> > 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
> >
>
> 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).
> I'm guessing that this is an issue because clang optimizes the static call
> site, right?
Specifically using Jcc (jne in this case) to tail-call the trampoline.
> > Teach the in-line static call text patching about this.
> >
> > Notably, since there is no conditional-ret, in that caes patch the Jcc
>
> "in that case"
typing so hard.. :-)
next prev parent reply other threads:[~2023-01-24 13:07 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-23 20:59 [PATCH 0/3] static_call/x86: Handle clang's conditional tail calls Peter Zijlstra
2023-01-23 20:59 ` [PATCH 1/3] x86/alternative: Introduce int3_emulate_jcc() Peter Zijlstra
2023-01-31 14:22 ` [tip: x86/alternatives] x86/alternatives: " tip-bot2 for Peter Zijlstra
2023-01-23 20:59 ` [PATCH 2/3] x86/alternative: Teach text_poke_bp() to patch Jcc.d32 instructions Peter Zijlstra
2023-01-31 14:22 ` [tip: x86/alternatives] x86/alternatives: " tip-bot2 for Peter Zijlstra
2023-01-23 20:59 ` [PATCH 3/3] x86/static_call: Add support for Jcc tail-calls Peter Zijlstra
2023-01-23 22:44 ` Steven Rostedt
2023-01-24 13:06 ` Peter Zijlstra [this message]
2023-01-24 15:07 ` Steven Rostedt
2023-01-26 15:34 ` [PATCH v1.1 " Peter Zijlstra
2023-01-26 18:14 ` Nick Desaulniers
2023-02-06 16:07 ` Steven Rostedt
2023-01-31 14:22 ` [tip: x86/alternatives] " tip-bot2 for Peter Zijlstra
2023-02-08 22:36 ` [PATCH 0/3] static_call/x86: Handle clang's conditional tail calls Nathan Chancellor
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