Re: [PATCH 3/4] x86/hvm: Remove cross-vendor checks from MSR handlers.

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

On Fri Jan 23, 2026 at 7:35 PM CET, Andrew Cooper wrote:
> On 22/01/2026 4:49 pm, Alejandro Vallejo wrote:
>> Not a functional change now that cross-vendor guests are not launchable.
>>
>> Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
>> ---
>>  xen/arch/x86/msr.c | 6 ++----
>>  1 file changed, 2 insertions(+), 4 deletions(-)
>>
>> diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
>> index ad75a2e108..c9cc4f0692 100644
>> --- a/xen/arch/x86/msr.c
>> +++ b/xen/arch/x86/msr.c
>> @@ -169,9 +169,9 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t 
>> *val)
>>          break;
>>  
>>      case MSR_IA32_PLATFORM_ID:
>> -        if ( !(cp->x86_vendor & X86_VENDOR_INTEL) ||
>> -             !(boot_cpu_data.x86_vendor & X86_VENDOR_INTEL) )
>> +        if ( cp->x86_vendor != X86_VENDOR_INTEL )
>>              goto gp_fault;
>> +
>>          rdmsrl(MSR_IA32_PLATFORM_ID, *val);
>>          break;
>>  
>> @@ -190,8 +190,6 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t 
>> *val)
>>           * the guest.
>>           */
>>          if ( !(cp->x86_vendor & (X86_VENDOR_INTEL | X86_VENDOR_AMD)) ||
>> -             !(boot_cpu_data.x86_vendor &
>> -               (X86_VENDOR_INTEL | X86_VENDOR_AMD)) ||
>>               rdmsr_safe(MSR_AMD_PATCHLEVEL, val) )
>>              goto gp_fault;
>>          break;
>
> Hmm.  Thinking about it, this would probably be cleaner to get rid of
> the cp->x86_vendor field entirely, and retain the boot_cpu_data side.
>
> Additionally, this would fix a minor problem I'm having cleaning up the
> CPUID code for XSAVE fixes, and provide better cache locality.
>
> ~Andrew

I'll replace it on the new version for consistency. I don't mind.

FYI, I'm planning to replace all occurences of this on a follow-up series with a
new cpu_vendor() helper (it's a simpler version evolved from the x86_vendor_is()
RFC I sent a while ago) to enhance DCE's ability to eliminate code. It is (I
hope) just a transient matter until all {x86_,}vendor field checks are replaced
by cpu_vendor().

Cheers,
Alejandro