Re: [PATCH v3 4/5] x86: Read MSR_ARCH_CAPS immediately after early_microcode_init()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.06.2023 17:48, Alejandro Vallejo wrote:
> --- a/xen/arch/x86/cpu/microcode/core.c
> +++ b/xen/arch/x86/cpu/microcode/core.c
> @@ -879,5 +879,18 @@ int __init early_microcode_init(unsigned long 
> *module_map,
>      if ( ucode_mod.mod_end || ucode_blob.size )
>          rc = early_microcode_update_cpu();
>  
> +    /*
> +     * We might have exposed MSR_ARCH_CAPS after the microcode update.

I'm struggling a little with this sentence, but not being a native speaker
it may be me, not the sentence. I would perhaps have said "MSR_ARCH_CAPS
may have appeared with the microcode update."

> +     * Reload relevant fields in boot_cpu_data if so because they are
> +     * needed in tsx_init()

Nit: Missing full stop.

I also wonder whether you wouldn't want to insert "e.g.", since iirc with
the next patch tsx_init() isn't going to be the only user anymore.

> +     */
> +    if ( boot_cpu_data.cpuid_level >= 7 )
> +        boot_cpu_data.x86_capability[FEATURESET_7d0]
> +            = cpuid_count_edx(7, 0);

I take it we assume the maximum CPUID level won't go from below 7 to 7
or higher with the ucode update?

> --- a/xen/arch/x86/tsx.c
> +++ b/xen/arch/x86/tsx.c
> @@ -39,9 +39,9 @@ void tsx_init(void)
>      static bool __read_mostly once;
>  
>      /*
> -     * This function is first called between microcode being loaded, and 
> CPUID
> -     * being scanned generally.  Read into boot_cpu_data.x86_capability[] for
> -     * the cpu_has_* bits we care about using here.
> +     * This function is first called between microcode being loaded, and
> +     * CPUID being scanned generally. early_microcode_init() has already
> +     * prepared the feature bits needed here after the microcode update.

Is this true in all cases? early_microcode_init() may have bailed
early, so I think you need to further transform early_microcode_init()
(and as a personal request of mine preferably without goto).

Jan