linux-kernel - Re: [PATCH 4/5] x86/fpu: Speed up lazy FPU restores slightly

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Date:	Sat, 23 Jan 2016 14:09:59 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	yu-cheng yu <yu-cheng.yu@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 4/5] x86/fpu: Speed up lazy FPU restores slightly

On Sat, Jan 23, 2016 at 2:14 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Fri, Jan 22, 2016 at 04:56:05PM -0800, Andy Lutomirski wrote:
>> If we have an FPU, there's no need to check CR0 for FPU emulation.
>>
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>>  arch/x86/kernel/traps.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>> index 87f80febf477..183b300f6a8b 100644
>> --- a/arch/x86/kernel/traps.c
>> +++ b/arch/x86/kernel/traps.c
>> @@ -752,7 +752,7 @@ do_device_not_available(struct pt_regs *regs, long error_code)
>>       RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
>>
>>  #ifdef CONFIG_MATH_EMULATION
>> -     if (read_cr0() & X86_CR0_EM) {
>> +     if (!cpu_has_fpu && (read_cr0() & X86_CR0_EM)) {
>
> Please use
>
>         boot_cpu_has(X86_FEATURE_FPU)
>
> We want to kill those cpu_has_xxx things.

Queued for v2.

Maybe this will be just cpu_has some day if my theory about the new
improved static_cpu_has being shorter than boot_cpu_has pans out :)

--Andy