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

[Ingo Molnar <mingo@kernel.org>]

* 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 | 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.

Thanks,

	Ingo