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 09:46:58 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC:     "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>,
        "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

Hi Rick,

Thanks for the review!

On Thu, Aug 24, 2023 at 06:16:55AM +0800, Edgecombe, Rick P wrote:
> 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.
 
Good point. It's also my first time to know that this cpuid bit could
change with CR4 runtime setting.

> > 
> > [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.
> 	 */

Will add. thanks!

- Feng