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

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Message-ID: <aCWjZ0LsNz8a7fjP@gmail.com>
Date: Thu, 15 May 2025 10:18:47 +0200
From: Ingo Molnar <mingo@...nel.org>
To: "Kirill A. Shutemov" <kirill@...temov.name>
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


* Kirill A. Shutemov <kirill@...temov.name> wrote:

> 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?

That's patch #7 :-)

I got confused about that too.

Patch #2 should not touch this line, and patch #7 should simply 
introduce __ro_after_init, and we are good I think.

Thanks,

	Ingo