Re: [PATCH v3 2/6] x86/microcode/core: Take a snapshot before and after applying microcode

[Dave Hansen <dave.hansen@intel.com>]

On 1/3/23 10:02, Ashok Raj wrote:
> The kernel caches features about each CPU's features at boot in an
> x86_capability[] structure. The microcode update takes one snapshot and
> compares it with the saved copy at boot.
> 
> However, the capabilities in the boot copy can be turned off as a result of
> certain command line parameters or configuration restrictions. This can
> cause a mismatch when comparing the values before and after the microcode
> update.
> 
> microcode_check() is called after an update to report any previously
> cached CPUID bits might have changed due to the update.
> 
> microcode_store_cpu_caps() basically stores the original CPU reported
> values and not the OS modified values. This will avoid giving a false
> warning even if no capabilities have changed.
> 
> Ignore the capabilities recorded at boot. Take a new snapshot before the
> update and compare with a snapshot after the update to eliminate the false
> warning.
...

It took me a moment to square this "Ignore the capabilities recorded at
boot." statement with the continued existence of:

	memcpy(info->x86_capability, &boot_cpu_data.x86_capability, ...

I think just adding "hardware" might help:

	Ignore all hardware capabilities recorded at boot.

Or even adding one more sentence:

	Only consult the synthetic capabilities recorded at boot so that
	a simple memcmp() can be used for comparisons.

> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 387578049de0..ac2e67156b9b 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -697,6 +697,7 @@ bool xen_set_default_idle(void);
>  #endif
>  
>  void __noreturn stop_this_cpu(void *dummy);
> +void microcode_store_cpu_caps(struct cpuinfo_x86 *info);
>  void microcode_check(struct cpuinfo_x86 *info);
>  
>  enum l1tf_mitigations {
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index b9c7529c920e..7c86c6fd07ae 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2297,28 +2297,43 @@ void cpu_init_secondary(void)
>  #endif
>  
>  #ifdef CONFIG_MICROCODE_LATE_LOADING
> +
> +void microcode_store_cpu_caps(struct cpuinfo_x86 *info)
> +{
> +	/* Reload CPUID max function as it might've changed. */
> +	info->cpuid_level = cpuid_eax(0);
> +
> +	/*
> +	 * Copy all capability leafs to pick up the synthetic ones so that
> +	 * memcmp() below doesn't fail on that. The ones coming from CPUID will
> +	 * get overwritten in get_cpu_cap().
> +	 */
> +	memcpy(info->x86_capability, &boot_cpu_data.x86_capability,
> +	       sizeof(info->x86_capability));
> +
> +	get_cpu_cap(info);
> +}
> +
>  /*
>   * The microcode loader calls this upon late microcode load to recheck features,
>   * only when microcode has been updated. Caller holds microcode_mutex and CPU
>   * hotplug lock.
>   */
> -void microcode_check(struct cpuinfo_x86 *info)
> +void microcode_check(struct cpuinfo_x86 *orig)
>  {
> -	perf_check_microcode();
> +	struct cpuinfo_x86 info;

'info' is kinda a throwaway name.  would this be better as:

	prev_info / new_info

instead of:

	orig / info

?

> -	/* Reload CPUID max function as it might've changed. */
> -	info->cpuid_level = cpuid_eax(0);
> +	perf_check_microcode();
>  
>  	/*
>  	 * Copy all capability leafs to pick up the synthetic ones so that
>  	 * memcmp() below doesn't fail on that. The ones coming from CPUID will
>  	 * get overwritten in get_cpu_cap().
>  	 */

This comment got copied to microcode_store_cpu_caps().  Does this
instance still need to be here?

> -	memcpy(&info->x86_capability, &boot_cpu_data.x86_capability, sizeof(info->x86_capability));
> -
> -	get_cpu_cap(info);
> +	microcode_store_cpu_caps(&info);
>  
> -	if (!memcmp(&info->x86_capability, &boot_cpu_data.x86_capability, sizeof(info->x86_capability)))
> +	if (!memcmp(&info.x86_capability, &orig->x86_capability,
> +		    sizeof(info.x86_capability)))
>  		return;
>  
>  	pr_warn("x86/CPU: CPU features have changed after loading microcode, but might not take effect.\n");
> diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
> index d86a4f910a6b..14d9031ed68a 100644
> --- a/arch/x86/kernel/cpu/microcode/core.c
> +++ b/arch/x86/kernel/cpu/microcode/core.c
> @@ -447,6 +447,13 @@ static int microcode_reload_late(void)
>  	atomic_set(&late_cpus_in,  0);
>  	atomic_set(&late_cpus_out, 0);
>  
> +	/*
> +	 * Take a snapshot before the microcode update, so we can compare
> +	 * them after the update is successful to check for any bits
> +	 * changed.
> +	 */
> +	microcode_store_cpu_caps(&info);

A "we" snuck in there.  How about this?

	/*
	 * Take a snapshot before the microcode update.  This enables
	 * a later comparison to see any bits changed after an update.
	 */

I do think some better naming of 'info' here would be nice too.
'old_info' or 'prev_info' seem like good alternatives.

>  	ret = stop_machine_cpuslocked(__reload_late, NULL, cpu_online_mask);
>  	if (ret == 0)
>  		microcode_check(&info);