Re: [PATCH v6 1/3] x86/microcode: Ignore microcode loading interface for revision = -1

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.07.2023 18:52, Alejandro Vallejo wrote:
> Some hypervisors report ~0 as the microcode revision to mean "don't issue
> microcode updates". Ignore the microcode loading interface in that case.
> 
> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Hmm.

> --- a/xen/arch/x86/cpu/microcode/core.c
> +++ b/xen/arch/x86/cpu/microcode/core.c
> @@ -867,10 +867,23 @@ int __init early_microcode_init(unsigned long 
> *module_map,
>          return -ENODEV;
>      }
>  
> -    microcode_grab_module(module_map, mbi);
> -
>      ucode_ops.collect_cpu_info();
>  
> +    /*
> +     * Some hypervisors deliberately report a microcode revision of -1 to
> +     * mean that they will not accept microcode updates. We take the hint
> +     * and ignore the microcode interface in that case.
> +     */
> +    if ( this_cpu(cpu_sig).rev == ~0 )
> +    {
> +        printk(XENLOG_INFO "Microcode loading disabled due to: %s",

While we have tentatively agreed to adjust what _will_ be emitted by
default (subject to suitable justification in that change's
description), such a patch is yet to be sent. As it stands this message
will be invisible by default.

> +                           "HW toggle");

With the comment talking about hypervisors, what is this string supposed
to tell an observer of the message in a log file?

Jan