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

[Ingo Molnar <mingo@kernel.org>]

* Brian Gerst <brgerst@gmail.com> wrote:

> On Tue, May 13, 2025 at 7:40 AM Ard Biesheuvel <ardb+git@google.com> wrote:
> >
> > From: Ard Biesheuvel <ardb@kernel.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@kernel.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.

Thanks,

	Ingo