Re: [PATCH] x86/cpufeature: Remove __pure attribute to _static_cpu_has()

From: hpa@zytor.com
To: Borislav Petkov <bp@alien8.de>, Nadav Amit <nadav.amit@gmail.com>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Damian Tometzki <linux_dti@icloud.com>,
	linux-integrity <linux-integrity@vger.kernel.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Linux-MM <linux-mm@kvack.org>, Will Deacon <will.deacon@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Kristen Carlson Accardi <kristen@linux.intel.com>,
	"Dock, Deneen T" <deneen.t.dock@intel.com>,
	Kees Cook <keescook@chromium.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: [PATCH] x86/cpufeature: Remove __pure attribute to _static_cpu_has()
Date: Thu, 07 Mar 2019 08:43:50 -0800	[thread overview]
Message-ID: <D683E00D-845E-4970-80DE-AD1DED3AE609@zytor.com> (raw)
In-Reply-To: <20190307151036.GD26566@zn.tnic>

On March 7, 2019 7:10:36 AM PST, Borislav Petkov <bp@alien8.de> wrote:
>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@suse.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@gmail.com>
>Signed-off-by: Borislav Petkov <bp@suse.de>
>Cc: Peter Zijlstra <peterz@infradead.org>
>Cc: x86@kernel.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"

Uhm... (a) it is correct, even if the compiler doesn't use it now, it allows the compiler to CSE it in the future; (b) it is documentation; (c) there is an actual bug here: the "volatile" implies a side effect, which in reality is not present, inhibiting CSE.

So the correct fix is to remove "volatile", not remove "__pure".
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.