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

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Message-ID: <87r0nsddb5.ffs@tglx>
Date:   Thu, 24 Aug 2023 11:01:18 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Feng Tang <feng.tang@...el.com>, 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
Cc:     Feng Tang <feng.tang@...el.com>
Subject: Re: [PATCH] x86/fpu: set X86_FEATURE_OSXSAVE feature after enabling
 OSXSAVE in CR4

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.

     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.

I fixed it up and added a proper comment explaining it.