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Re: file xen/include/xen/lib/x86/cpu-policy.h: Meaning of CPUID constants
- To: Fonyuy-Asheri Caleb <fonyuy-asheri.caleb@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 6 May 2024 10:13:42 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 06 May 2024 08:13:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 06.05.2024 09:46, Fonyuy-Asheri Caleb wrote:
> I came across these constants in the code
> (xen/include/xen/lib/x86/cpu-policy.h file) but no explanation of why they
> have been set that way.
>
> #define CPUID_GUEST_NR_BASIC (0xdu + 1)
> #define CPUID_GUEST_NR_CACHE (5u + 1)
> #define CPUID_GUEST_NR_FEAT (2u + 1)
> #define CPUID_GUEST_NR_TOPO (1u + 1)
> #define CPUID_GUEST_NR_XSTATE (62u + 1)
> #define CPUID_GUEST_NR_EXTD_INTEL (0x8u + 1)
> #define CPUID_GUEST_NR_EXTD_AMD (0x21u + 1)
>
> Please can someone explain to me why we have these constants or point to a
> documentation which explains it?
These specify the number of (sub)leaves Xen supports, first and foremost to
dimension internal arrays accordingly. I.e. the "why" lies in the way they're
used.
> I am particularly interested in the CPUID_GUEST_NR_BASIC given that for intel
> processors for example, we have
> basic leaves running up to 0x21u already for recent processors. This value
> sort of forces a particular max leaf value.
Right, but support (for guests) for these needs properly enabling. Hence why
that limit is the way it is, for the time being.
Jan
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