[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: file xen/include/xen/lib/x86/cpu-policy.h: Meaning of CPUID constants
> From: "Roger Pau Monné" <roger.pau@xxxxxxxxxx> > To: "Fonyuy-Asheri Caleb" <fonyuy-asheri.caleb@xxxxxxxx> > Cc: "xen-devel" <xen-devel@xxxxxxxxxxxxxxxxxxxx> > Sent: Monday, May 6, 2024 10:34:20 AM > Subject: Re: file xen/include/xen/lib/x86/cpu-policy.h: Meaning of CPUID > constants > On Mon, May 06, 2024 at 09:46:58AM +0200, Fonyuy-Asheri Caleb wrote: >> Hi, >> I am currently doing a study on the way xen handles CPUID information. >> >> 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? >> 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. > > This is related to the maximum leaves supported in the cpu_policy > structure: > > https://elixir.bootlin.com/xen/latest/source/xen/include/xen/lib/x86/cpu-policy.h#L122 > > For basic leaves (0x000000xx) we support up to leaf 0xd (XSTATE). It > doesn't mean there are no further leaves behind it, but we likely > still have no use for them, and hence they are not part of the policy. > The cpu-policy is used to store a (cpuid) policy object for guests, > so if the information exposed in those leaves are related to features > that are never exposed to guests is makes no sense to have space for > them. So if I understand you well, you mean before extending this, we need to perform a study on the leaves to confirm how useful they are to the guests depending on the cpu information they carry. > > Regards, Roger.
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