linux-kernel - [PATCH] x86/cpufeature: Remove __pure attribute to _static_cpu

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Message-ID: <20190307151036.GD26566@zn.tnic>
Date:   Thu, 7 Mar 2019 16:10:36 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Damian Tometzki <linux_dti@...oud.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux-MM <linux-mm@...ck.org>, Will Deacon <will.deacon@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        "Dock, Deneen T" <deneen.t.dock@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: [PATCH] x86/cpufeature: Remove __pure attribute to _static_cpu_has()

On Mon, Feb 11, 2019 at 12:32:41PM -0800, Nadav Amit wrote:
> BTW: the “__pure” attribute is useless when “__always_inline” is used.
> Unless it is intended to be some sort of comment, of course.

---
From: Borislav Petkov <bp@...e.de>
Date: Thu, 7 Mar 2019 15:54:51 +0100

__pure is used to make gcc do Common Subexpression Elimination (CSE)
and thus save subsequent invocations of a function which does a complex
computation (without side effects). As a simple example:

  bool a = _static_cpu_has(x);
  bool b = _static_cpu_has(x);

gets turned into

  bool a = _static_cpu_has(x);
  bool b = a;

However, gcc doesn't do CSE with asm()s when those get inlined - like it
is done with _static_cpu_has() - because, for example, the t_yes/t_no
labels are different for each inlined function body and thus cannot be
detected as equivalent anymore for the CSE heuristic to hit.

However, this all is beside the point because best it should be avoided
to have more than one call to _static_cpu_has(X) in the same function
due to the fact that each such call is an alternatives patch site and it
is simply pointless.

Therefore, drop the __pure attribute as it is not doing anything.

Reported-by: Nadav Amit <nadav.amit@...il.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org
---
 arch/x86/include/asm/cpufeature.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index e25d11ad7a88..6d6d5cc4302b 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -162,7 +162,7 @@ extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
  * majority of cases and you should stick to using it as it is generally
  * only two instructions: a RIP-relative MOV and a TEST.
  */
-static __always_inline __pure bool _static_cpu_has(u16 bit)
+static __always_inline bool _static_cpu_has(u16 bit)
 {
 	asm_volatile_goto("1: jmp 6f\n"
 		 "2:\n"
-- 
2.21.0

-- 
Regards/Gruss,
    Boris.

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