linux-kernel - Re: [RFC PATCH v2 3/6] x86/cpu: Allow caps to be set arbitrarily early

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Message-ID: <CAMj1kXHzka4hXXE8UU9MvPnuqa1Jd0XBiNP8FGkj6Oc9J5JaWQ@mail.gmail.com>
Date: Wed, 14 May 2025 10:49:22 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Brian Gerst <brgerst@...il.com>, Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	x86@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH v2 3/6] x86/cpu: Allow caps to be set arbitrarily early

On Wed, 14 May 2025 at 09:17, Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * Brian Gerst <brgerst@...il.com> wrote:
>
> > On Tue, May 13, 2025 at 7:40 AM 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 | 10 ++++------
> > >  1 file changed, 4 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> > > index bbec5c4cd8ed..aaa6d9e51ef1 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));
> >
> > Is there any scenario where capabilities are changed after boot?
>
> Not supposed to...
>
> > If not, this could possibly be __ro_after_init.
>
> Yeah, and in a separate patch.
>

OK.