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

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Date:   Wed, 23 Aug 2023 22:16:55 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Hansen, Dave" <dave.hansen@...el.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Sang, Oliver" <oliver.sang@...el.com>,
        "Tang, Feng" <feng.tang@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/fpu: set X86_FEATURE_OSXSAVE feature after enabling
 OSXSAVE in CR4

On Wed, 2023-08-23 at 14:57 +0800, Feng Tang wrote:
> 0Day found a 34.6% regression in stress-ng's 'af-alg' test case [1],
> and
> bisected it to commit b81fac906a8f ("x86/fpu: Move FPU initialization
> into arch_cpu_finalize_init()"), which optimizes the FPU init order,
> and moves the CR4_OSXSAVE enabling into a later place:
> 
>    arch_cpu_finalize_init
>        identify_boot_cpu
>            identify_cpu
>                generic_identify
>                    get_cpu_cap --> setup cpu capability
>        ...
>        fpu__init_cpu
>            fpu__init_cpu_xstate
>                cr4_set_bits(X86_CR4_OSXSAVE);
> 
> And it makes 'X86_FEATURE_OSXSAVE' feature bit missed in cpu
> capability
> setup. Many security module like 'camellia_aesni_avx_x86_64' depends
> on
> this feature, and will fail to be loaded after the commit, causing
> the
> regression.
> 
> So set X86_FEATURE_OSXSAVE feature right after OSXSAVE enabling to
> fix it.

Oh, that's unfortunate.

It might help to include a bit more about the problem in the log. The
piece that confused me at first was that X86_FEATURE_OSXSAVE maps to a
CPUID bit that will change once CR4.OSXSAVE is set. So since the CPUID
bits are read before CR4.OSXSAVE is set, it stores the original unset
value of the bit.

> 
> [1].
> https://lore.kernel.org/lkml/202307192135.203ac24e-oliver.sang@intel.com/
> 
> Fixes: b81fac906a8f ("x86/fpu: Move FPU initialization into
> arch_cpu_finalize_init()")
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Signed-off-by: Feng Tang <feng.tang@...el.com>
> ---
>  arch/x86/kernel/fpu/xstate.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> 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);
> +

I'd also put a comment here to explain why this is done manually. I'll
toss something out in case it's useful:
	/*
	 * CPUID bit for X86_FEATURE_OSXSAVE value will change once 
	 * CR4.OSXSAVE is set, so update it manually.
	 */