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

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Message-ID: <aCXZOTi6Oe0jiNq2@gmail.com>
Date: Thu, 15 May 2025 14:08:25 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: "Kirill A. Shutemov" <kirill@...temov.name>,
	Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org,
	x86@...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


* Ard Biesheuvel <ardb@...nel.org> wrote:

> On Thu, 15 May 2025 at 09:18, Ingo Molnar <mingo@...nel.org> wrote:
> >
> >
> > * 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.
> >
> 
> This change is needed because it prevents these arrays from being
> cleared along with the rest of BSS, which occurs after the startup
> code executes.

I see, it's a correctness & bisectability aspect, even if it's 
obsoleted later on. Good point, objection withdrawn.

> So conceptually, moving these out of BSS is similar to dropping the
> memset()s, and therefore this belongs in the same patch.
> 
> However, you are correct that moving these into __ro_after_init
> achieves the same thing, so I will just reorder that patch with this
> one, and clarify in the commit log that we are relying on the fact
> that __ro_after_init is not cleared at boot time.

That works fine for me too - whichever order you prefer.

Thanks,

	Ingo