Help With Identifying CPUID faulting logic in Xen code

Help With Identifying CPUID faulting logic in Xen code

[Fonyuy-Asheri Caleb]

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Hello, 

I am interested in finding understanding how xen handles CPUID faulting and 
VM exits in general. Please can someone indicate to me the concerned files? 

I want to know how xen detects the execution of the CPUID instruction and 
ensures a guest only gets the features defined in cpuid-autogen.h file 
depending on the guest type. 

I started looking at the file xen/arch/x86/cpuid.c but don't really know which other 
files to check next. 

Thanks 

Caleb 

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Re: Help With Identifying CPUID faulting logic in Xen code

[Andrew Cooper]

On 03/01/2025 2:25 pm, Fonyuy-Asheri Caleb wrote:
> Hello,
>
> I am interested in finding understanding how xen handles CPUID
> faulting and 
> VM exits in general. Please can someone indicate to me the concerned
> files? 
>
> I want to know how xen detects the execution of the CPUID instruction and 
> ensures a guest only gets the features defined in cpuid-autogen.h file 
> depending on the guest type. 
>
> I started looking at the file xen/arch/x86/cpuid.c but don't really
> know which other 
> files to check next.

https://xenbits.xen.org/docs/unstable/features/feature-levelling.html

has a reasonable introduction.  Being nearly a decade old, it's slightly
stale.  AMD now have CPUID Faulting capability too, new in Zen4 CPUs IIRC.

LCAP_faulting specifically is the constant you're looking for, in the
Xen code.

~Andrew

Re: Help With Identifying CPUID faulting logic in Xen code

[Fonyuy-Asheri Caleb]

Thank you

Caleb

----- Original Message -----
> From: "Andrew Cooper" <andrew.cooper3@citrix.com>
> To: "Fonyuy-Asheri Caleb" <fonyuy-asheri.caleb@inria.fr>, "xen-devel" <xen-devel@lists.xenproject.org>
> Cc: "Jan Beulich" <jbeulich@suse.com>
> Sent: Friday, January 3, 2025 3:35:43 PM
> Subject: Re: Help With Identifying CPUID faulting logic in Xen code

> On 03/01/2025 2:25 pm, Fonyuy-Asheri Caleb wrote:
>> Hello,
>>
>> I am interested in finding understanding how xen handles CPUID
>> faulting and
>> VM exits in general. Please can someone indicate to me the concerned
>> files?
>>
>> I want to know how xen detects the execution of the CPUID instruction and
>> ensures a guest only gets the features defined in cpuid-autogen.h file
>> depending on the guest type.
>>
>> I started looking at the file xen/arch/x86/cpuid.c but don't really
>> know which other
>> files to check next.
> 
> https://xenbits.xen.org/docs/unstable/features/feature-levelling.html
> 
> has a reasonable introduction.  Being nearly a decade old, it's slightly
> stale.  AMD now have CPUID Faulting capability too, new in Zen4 CPUs IIRC.
> 
> LCAP_faulting specifically is the constant you're looking for, in the
> Xen code.
> 
> ~Andrew