From: Fonyuy-Asheri Caleb <fonyuy-asheri.caleb@inria.fr>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: file xen/include/xen/lib/x86/cpu-policy.h: Meaning of CPUID constants
Date: Mon, 6 May 2024 10:45:59 +0200 (CEST) [thread overview]
Message-ID: <1049993614.4894172.1714985159910.JavaMail.zimbra@inria.fr> (raw)
In-Reply-To: <ZjiWDMXFNmwSzAxQ@macbook>
> From: "Roger Pau Monné" <roger.pau@citrix.com>
> To: "Fonyuy-Asheri Caleb" <fonyuy-asheri.caleb@inria.fr>
> Cc: "xen-devel" <xen-devel@lists.xenproject.org>
> 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.
next prev parent reply other threads:[~2024-05-06 8:46 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-06 7:46 file xen/include/xen/lib/x86/cpu-policy.h: Meaning of CPUID constants Fonyuy-Asheri Caleb
2024-05-06 8:13 ` Jan Beulich
2024-05-06 8:34 ` Roger Pau Monné
2024-05-06 8:45 ` Fonyuy-Asheri Caleb [this message]
2024-05-06 8:47 ` Jan Beulich
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