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

[Fouad Hilly <fouad.hilly@xxxxxxxxx>]

On Mon, May 6, 2024 at 9:45 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> 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?

The checking of the microcode signature and if it is applicable to the
CPU, will be at vendor code level i.e Intel\AMD.
"min_rev" mention to be removed. This change is for microcode
downgrade capability.

>
> > 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.
>

Noted and will be fixed in V4

> > --- 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?

Will be fixed in V4

Fouad


>
> 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