RE: [Xen-devel] [PATCH]: Add support for Intel CPUID Feature mask in Xen

["Ke, Liping" <liping.ke@xxxxxxxxx>]

Hi, Ian
See comments below:
Thanks& Regards,
Criping

Ian Pratt wrote:
>> Hi, all
>> Some Latest Intel CPU models (CPUID.1.EAX >= 0x00010674) support
>> cupid mask features. Cpuid feature mask is used to limit the feature
>> flags reported by CPUID.1.EDX:ECX. 
>> 
>> This small patch adds CPUID feature mask support in XEN by adding
>> boot options in Grub entry as below example:
>> kernel /boot/xen.gz com1=115200,8n1 console=com1 cpuid_mask_ecx =
>> 0xFFFF7FFF cpuid_mask_edx = 0xFFFFFFFC
>> 
>> For detailed usage information, please refer to the attached
>> document: cupid_feature_mask.txt.
> 
> It's good to see this feature enabled, but it would be a lot more user
> friendly if the patch took a cpu family and model number and then
> 'flattened' the cpuid back to the specification of that CPU. We don't
> need to go back very far (certainly not pre-VT), and I'd be happy if
> we just did all core2 (and newer) CPUs.
> 
We also hesitate how to solve it for some time. Yet we found since cpu 
models are quite much even from core2, finding each of them is not that easy
(cpuid.1.ECX:EDX might only could be referred in corresponding BIOS guide). 

Also, if using one older type that we did not cover, then there's some problem. 
What's more, ecx/edx only give few features that  users might do care. 
So maybe users should know by themselves on how to set the few bits 
to 0 and passes others just 1.

So we decided just give the mask and give users detailed docs help?

> Also, is there a magic MSR to change the CPU family/model number
> reported? It would make sense to do this too.
Oh, No, there are no such magic MSR. Only cpuid.1.ECX:EDX could be masked.

> 
> At what point does the CPU masking occur? Ideally we'd do it just
> before starting dom0. [Actually, we might want to be able to invoke
> it from libxc in future too]
When setting cpuid_mask, both HV and all domains will all be cheated from this
point, it is a hardware and-logic .
So for consistency, we set the msr from xen start time after detecting cpu. 
Seems setting@xen startup Or setting@domain0 startup 
will not make a big difference here?

At earlier code, we add switch code when doing domain scheduling, 
only those pv_guest(also apps) who defines this cpuid_mask will be cheated. 
When domain0/HV is scheduled, we Will reset cpuid_mask back to 0xFFFFFFFF. 
But later we thought this price is too big and Code is not clean, so we give it 
up.

> 
> Ian


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