Re: [RFC PATCH] x86/cpufeature: Add feature dependency checks

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 22, 2024, Sohil Mehta wrote:
> Currently, the cpuid-deps[] table is only exercised when a particular
> feature gets explicitly disabled and clear_cpu_cap() is called. However,
> some of these listed dependencies might already be missing during boot.
> Unexpected failures can occur when the kernel tries to use such a
> feature.
> 
> Therefore, add boot time checks for missing feature dependencies and
> disable any feature whose dependencies are not met.
> 
> Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
> ---
> Arguably, this situation should only happen on broken hardware and it may not
> make sense to add such a check to the kernel. OTOH, this can be viewed as a
> safety mechanism to make failures more graceful on such configurations in real
> or virtual environments.

And goofy Kconfigs.   But yeah, lack of any meaningful fallout is why my version
didn't go anywhere.

https://lore.kernel.org/all/20221203003745.1475584-2-seanjc@google.com

> I feel since we already have the cpuid-deps[] table and the incremental changes
> are small, this patch might be a useful addition.
> 
> Also, if this check seems worthwhile, would it be useful to combine and rewrite
> it with filter_cpuid_features() since it tries to do something similar?
> ---
> 
>  arch/x86/include/asm/cpufeature.h |  1 +
>  arch/x86/kernel/cpu/common.c      |  4 ++++
>  arch/x86/kernel/cpu/cpuid-deps.c  | 10 ++++++++++
>  3 files changed, 15 insertions(+)
> 
> diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
> index 0b9611da6c53..347ef04f65ef 100644
> --- a/arch/x86/include/asm/cpufeature.h
> +++ b/arch/x86/include/asm/cpufeature.h
> @@ -148,6 +148,7 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
>  
>  extern void setup_clear_cpu_cap(unsigned int bit);
>  extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
> +extern void filter_feature_dependencies(struct cpuinfo_x86 *c);
>  
>  #define setup_force_cpu_cap(bit) do {			\
>  							\
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index d4e539d4e158..6b725dbd8db7 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1602,6 +1602,7 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>  
>  		c->cpu_index = 0;
>  		filter_cpuid_features(c, false);
> +		filter_feature_dependencies(c);
>  
>  		if (this_cpu->c_bsp_init)
>  			this_cpu->c_bsp_init(c);
> @@ -1854,6 +1855,9 @@ static void identify_cpu(struct cpuinfo_x86 *c)
>  	/* Filter out anything that depends on CPUID levels we don't have */
>  	filter_cpuid_features(c, true);
>  
> +	/* Filter out features that don't have their dependencies met */
> +	filter_feature_dependencies(c);
> +
>  	/* If the model name is still unset, do table lookup. */
>  	if (!c->x86_model_id[0]) {
>  		const char *p;
> diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
> index b7d9f530ae16..88b34a97278a 100644
> --- a/arch/x86/kernel/cpu/cpuid-deps.c
> +++ b/arch/x86/kernel/cpu/cpuid-deps.c
> @@ -147,3 +147,13 @@ void setup_clear_cpu_cap(unsigned int feature)
>  {
>  	do_clear_cpu_cap(NULL, feature);
>  }
> +
> +void filter_feature_dependencies(struct cpuinfo_x86 *c)
> +{
> +	const struct cpuid_dep *d;
> +
> +	for (d = cpuid_deps; d->feature; d++) {
> +		if (boot_cpu_has(d->feature) && !boot_cpu_has(d->depends))

I don't think checking boot_cpu_has() is correct, it's entirely possible for a CPU
to have divergent features from the boot CPU, e.g. if a feature is dependent on
BIOS enabling (or disabling) and BIOS messed up.

> +			do_clear_cpu_cap(c, d->feature);
> +	}
> +}
> -- 
> 2.34.1
>