linux-kernel - Re: [PATCH v3 2/7] x86/cpu: Allow caps to be set arbitrarily early

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Message-ID: <u4abxvlhfrg4pdvtsej6zh2wizb7krg5okps347uwp5bhselwp@7e2cbs5scxpr>
Date: Thu, 15 May 2025 10:55:23 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Ingo Molnar <mingo@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH v3 2/7] x86/cpu: Allow caps to be set arbitrarily early

On Thu, May 15, 2025 at 08:56:59AM +0200, Ingo Molnar wrote:
> 
> * Ard Biesheuvel <ardb+git@...gle.com> wrote:
> 
> > From: Ard Biesheuvel <ardb@...nel.org>
> > 
> > cpu_feature_enabled() uses a ternary alternative, where the late variant
> > is based on code patching and the early variant accesses the capability
> > field in boot_cpu_data directly.
> > 
> > This allows cpu_feature_enabled() to be called quite early, but it still
> > requires that the CPU feature detection code runs before being able to
> > rely on the return value of cpu_feature_enabled().
> > 
> > This is a problem for the implementation of pgtable_l5_enabled(), which
> > is based on cpu_feature_enabled(X86_FEATURE_5LEVEL_PAGING), and may be
> > called extremely early. Currently, there is a hacky workaround where
> > some source files that may execute before (but also after) CPU feature
> > detection have a different version of pgtable_l5_enabled(), based on the
> > USE_EARLY_PGTABLE_L5 preprocessor macro.
> > 
> > Instead, let's make it possible to set CPU feature arbitrarily early, so
> > that the X86_FEATURE_5LEVEL_PAGING capability can be set before even
> > entering C code.
> > 
> > This involves relying on static initialization of boot_cpu_data and the
> > cpu_caps_set/cpu_caps_cleared arrays, so they all need to reside in
> > .data. This ensures that they won't be cleared along with the rest of
> > BSS.
> > 
> > Note that forcing a capability involves setting it in both
> > boot_cpu_data.x86_capability[] and cpu_caps_set[].
> > 
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> >  arch/x86/kernel/cpu/common.c | 9 +++------
> >  1 file changed, 3 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> > index 6f7827015834..f6f206743d6a 100644
> > --- a/arch/x86/kernel/cpu/common.c
> > +++ b/arch/x86/kernel/cpu/common.c
> > @@ -704,8 +704,8 @@ static const char *table_lookup_model(struct cpuinfo_x86 *c)
> >  }
> >  
> >  /* Aligned to unsigned long to avoid split lock in atomic bitmap ops */
> > -__u32 cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
> > -__u32 cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
> > +__u32 __read_mostly cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
> > +__u32 __read_mostly cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
> 
> This change is not mentioned in the changelog AFAICS, but it should be 
> in a separate patch anyway.

And why not __ro_after_init?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov