Re: [PATCH] x86/ucode: Further fixes to identify "ucode already up to date"

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 16/05/2024 12:50 pm, Roger Pau Monné wrote:
> On Thu, May 16, 2024 at 12:31:03PM +0100, Andrew Cooper wrote:
>> When the revision in hardware is newer than anything Xen has to hand,
>> 'microcode_cache' isn't set up.  Then, `xen-ucode` initiates the update
>> because it doesn't know whether the revisions across the system are symmetric
>> or not.  This involves the patch getting all the way into the
>> apply_microcode() hooks before being found to be too old.
>>
>> This is all a giant mess and needs an overhaul, but in the short term simply
>> adjust the apply_microcode() to return -EEXIST.
>>
>> Also, unconditionally print the preexisting microcode revision on boot.  It's
>> relevant information which is otherwise unavailable if Xen doesn't find new
>> microcode to use.
>>
>> Fixes: 648db37a155a ("x86/ucode: Distinguish "ucode already up to date"")
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> CC: Fouad Hilly <fouad.hilly@xxxxxxxxx>
>>
>> Sorry Fouad, but this collides with your `--force` series once again.
>> Hopefully it might make things fractionally easier.
>>
>> Background: For 06-55-04 (Skylake server, stepping 4 specifically), there's a
>> recent production firmware update which has a newer microcode revision than
>> exists in the Intel public microcode repository.  It's causing a mess in our
>> automated testing, although it is finding good bugs...
>> ---
>>  xen/arch/x86/cpu/microcode/amd.c   | 7 +++++--
>>  xen/arch/x86/cpu/microcode/core.c  | 2 ++
>>  xen/arch/x86/cpu/microcode/intel.c | 7 +++++--
>>  3 files changed, 12 insertions(+), 4 deletions(-)
>>
>> diff --git a/xen/arch/x86/cpu/microcode/amd.c 
>> b/xen/arch/x86/cpu/microcode/amd.c
>> index 17e68697d5bf..f76a563c8b84 100644
>> --- a/xen/arch/x86/cpu/microcode/amd.c
>> +++ b/xen/arch/x86/cpu/microcode/amd.c
>> @@ -222,12 +222,15 @@ static int cf_check apply_microcode(const struct 
>> microcode_patch *patch)
>>      uint32_t rev, old_rev = sig->rev;
>>      enum microcode_match_result result = microcode_fits(patch);
>>  
>> +    if ( result == MIS_UCODE )
>> +        return -EINVAL;
>> +
>>      /*
>>       * Allow application of the same revision to pick up SMT-specific 
>> changes
>>       * even if the revision of the other SMT thread is already up-to-date.
>>       */
>> -    if ( result != NEW_UCODE && result != SAME_UCODE )
>> -        return -EINVAL;
>> +    if ( result == OLD_UCODE )
>> +        return -EEXIST;
> Won't it be simpler to just add this check ahead of the existing one,
> so that you can leave the code as-is, iow:
>
>     if ( result == OLD_UCODE )
>         return -EEXIST;
>
>     /*
>      * Allow application of the same revision to pick up SMT-specific changes
>      * even if the revision of the other SMT thread is already up-to-date.
>      */
>     if ( result != NEW_UCODE && result != SAME_UCODE )
>         return -EINVAL;
>
> Thanks, Roger.

Not really, no.  That still leaves this piece of logic which is
misleading IMO.

MIS_UCODE is the only -EINVAL worthy case.

Every other *_UCODE constant needs to be 0 or -EEXIST, depending on
allow-same/--force.

~Andrew