linux-kernel - Re: [PATCH] x86/fpu: set X86_FEATURE_OSXSAVE feature after enabling OSXSAVE in CR4

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Date:   Thu, 24 Aug 2023 19:58:42 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Peter Zijlstra" <peterz@...radead.org>, <x86@...nel.org>,
        kernel test robot <oliver.sang@...el.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/fpu: set X86_FEATURE_OSXSAVE feature after enabling
 OSXSAVE in CR4

Hi Thomas,

On Thu, Aug 24, 2023 at 11:01:18AM +0200, Thomas Gleixner wrote:
> On Wed, Aug 23 2023 at 14:57, Feng Tang wrote:
> > diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> > index 0bab497c9436..8ebea0d522d2 100644
> > --- a/arch/x86/kernel/fpu/xstate.c
> > +++ b/arch/x86/kernel/fpu/xstate.c
> > @@ -173,6 +173,9 @@ void fpu__init_cpu_xstate(void)
> >  
> >  	cr4_set_bits(X86_CR4_OSXSAVE);
> >  
> > +	if (!boot_cpu_has(X86_FEATURE_OSXSAVE))
> > +		setup_force_cpu_cap(X86_FEATURE_OSXSAVE);
> 
> This is wrong in several aspects:
> 
>      1) You force the feature bit _before_ XSAVE is completely
>         initialized. fpu__init_system_xstate() has error paths which
>         disable XSAVE.

Yes, I missed the error path in BSP fpu initialization code.

>      2) This conditional should have been a red flag for you simply
>         because fpu__init_cpu_xstate() is invoked on all CPUs not only
>         on the BSP.

Indeed. when I worked on the patch, I even thought about ugly thing like: 

    if (raw_smp_processor_id() == 0)
	setup_force_cpu_cap(X86_FEATURE_OSXSAVE);
 
> I fixed it up and added a proper comment explaining it.

Thanks for fixing it up and improving the comments!

- Feng