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

From: hpa
Date: Thu Mar 07 2019 - 11:44:52 EST

On March 7, 2019 7:10:36 AM PST, Borislav Petkov <bp@xxxxxxxxx> wrote:
>On Mon, Feb 11, 2019 at 12:32:41PM -0800, Nadav Amit wrote:
>> BTW: the â__pureâ attribute is useless when â__always_inlineâ is
>> Unless it is intended to be some sort of comment, of course.
>From: Borislav Petkov <bp@xxxxxxx>
>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
>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
>is simply pointless.
>Therefore, drop the __pure attribute as it is not doing anything.
>Reported-by: Nadav Amit <nadav.amit@xxxxxxxxx>
>Signed-off-by: Borislav Petkov <bp@xxxxxxx>
>Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>Cc: x86@xxxxxxxxxx
> arch/x86/include/asm/cpufeature.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>diff --git a/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".
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