Re: [PATCH] x86/MCE: restore CPU vendor reporting to the outside world

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.02.2026 13:36, Andrew Cooper wrote:
> On 16/02/2026 11:56 am, Jan Beulich wrote:
>> The records reported used to contain Linux enumerators. We first broke
>> that connection when purging unused ones, and then again when switching to
>> the bit mask forms.
>>
>> Fixes: 408413051144 ("x86/cpu: Drop unused X86_VENDOR_* values")
>> Fixes: 0cd074144cbb ("x86/cpu: Renumber X86_VENDOR_* to form a bitmap")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> I see you've committed this, but it's not a viable approach.

Well, depends on what the goal is, I suppose: It's viable to restore previously
regressed behavior. We have been in agreement, I think, that vMCE itself is
doomed.

>> --- a/xen/arch/x86/cpu/mcheck/mce.c
>> +++ b/xen/arch/x86/cpu/mcheck/mce.c
>> @@ -858,6 +858,22 @@ void *x86_mcinfo_reserve(struct mc_info
>>      return mic_index;
>>  }
>>  
>> +/* Linux values need using when reporting CPU vendors to the outside. */
>> +static unsigned int xen2linux_vendor(unsigned int vendor)
>> +{
>> +    switch ( vendor )
>> +    {
>> +    case X86_VENDOR_INTEL:    return 0;
>> +    case X86_VENDOR_AMD:      return 2;
>> +    case X86_VENDOR_CENTAUR:  return 5;
>> +    case X86_VENDOR_HYGON:    return 9;
>> +    case X86_VENDOR_SHANGHAI: return 10; /* X86_VENDOR_ZHAOXIN */
>> +    default: break;
>> +    }
>> +
>> +    return 0xff; /* X86_VENDOR_UNKNOWN */
>> +}
> 
> The Linux x86 maintainers say that the numbers are internal and not an
> ABI, even they haven't changed since 2008.

Yet the numbers are propagated to the outside world there as well, aiui.

>> @@ -866,7 +882,7 @@ static void x86_mcinfo_apei_save(
>>      memset(&m, 0, sizeof(struct mce));
>>  
>>      m.cpu = mc_global->mc_coreid;
>> -    m.cpuvendor = boot_cpu_data.x86_vendor;
>> +    m.cpuvendor = xen2linux_vendor(boot_cpu_data.x86_vendor);
> 
> This is an internal Xen structure.  This hunk needs reverting.

It's not, unless I'm missing something. The struct is passed to
apei_write_mce(), to then be passed to erst_write(). ERST isn't Xen-
internal anymore, is it?

Jan