Re: [PATCH v3 1/5] x86: Update x86 low level version check of microcode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.04.2024 14:47, Fouad Hilly wrote:
> Update microcode version check at Intel and AMD Level by:
> Preventing the low level code from sending errors if the microcode
> version is not a newer version. this is required to allow developers to do
> ucode loading testing, including the introduction of Intel "min rev" field,
> which requires an override mechanism for newer version checks.

Won't "min rev" checking, for being Intel-only, require quite the opposite,
i.e. leaving more of the checking to vendor specific code?

> Even though
> the check for newer is removed at this level, it still exists at higher
> common level, where by default only newer version will be processed.
> The option to override version check, will be added as part of this patch
> series.

Please avoid wording like "this patch", "this commit", and all the more
"this patch series". Especially this last one will become completely
meaningless once part of a commit message in the tree.

> --- a/xen/arch/x86/cpu/microcode/amd.c
> +++ b/xen/arch/x86/cpu/microcode/amd.c
> @@ -384,11 +384,10 @@ static struct microcode_patch *cf_check 
> cpu_request_microcode(
>              }
>  
>              /*
> -             * If the new ucode covers current CPU, compare ucodes and store 
> the
> -             * one with higher revision.
> +             * If the microcode covers current CPU, then store its
> +             * revision.
>               */

Nit: This can become a single line comment now, can't it? (Again then in Intel
code.)

> --- a/xen/arch/x86/cpu/microcode/intel.c
> +++ b/xen/arch/x86/cpu/microcode/intel.c
> @@ -294,8 +294,7 @@ static int cf_check apply_microcode(const struct 
> microcode_patch *patch)
>  
>      result = microcode_update_match(patch);
>  
> -    if ( result != NEW_UCODE &&
> -         !(opt_ucode_allow_same && result == SAME_UCODE) )
> +    if ( result == MIS_UCODE )
>          return -EINVAL;

I continue to be in trouble with this change, despite the v3 adjustment
you make: If this is needed here, why would a similar change not be needed
for AMD?

Plus the original question remains: Is this actually valid to be changed
right here? IOW despite the somewhat improved patch description I'm still
having difficulty identifying which vendor-independent check this is
redundant with. As opposed to the AMD change further up and ...

> @@ -355,11 +354,10 @@ static struct microcode_patch *cf_check 
> cpu_request_microcode(
>              break;
>  
>          /*
> -         * If the new update covers current CPU, compare updates and store 
> the
> -         * one with higher revision.
> +         * If the microcode covers current CPU, then store its
> +         * revision.
>           */
> -        if ( (microcode_update_match(mc) != MIS_UCODE) &&
> -             (!saved || compare_revisions(saved->rev, mc->rev) == NEW_UCODE) 
> )
> +        if ( (microcode_update_match(mc) != MIS_UCODE) && !saved )
>              saved = mc;

... this one, where I can see that they are about caching of ucode blobs,
which looks to be dealt with by cache maintenance logic in
microcode_update_helper() and microcode_update_cache().

Jan