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Message-ID: <Zf+uKi42qFoHLAUg@gmail.com>
Date: Sun, 24 Mar 2024 05:38:02 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, jgross@...e.com, tglx@...utronix.de,
x86@...nel.org, bp@...en8.de
Subject: Re: [PATCH 1/4] x86/cpu: Add and use new CPUID region helper
* Dave Hansen <dave.hansen@...ux.intel.com> wrote:
>
> From: Dave Hansen <dave.hansen@...ux.intel.com>
>
> There are some (before now) unwritten rules about CPUID "regions".
> Basically, there is a 32-bit address space of CPUID leaves. The
> top 16 bits address a "region" and the first leaf in a region
> is special.
>
> The kernel only has a few spots that care about this, but it's
> rather hard to make sense of the code as is.
>
> Add a helper that explains regions. Use it where applicable.
>
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Juergen Gross <jgross@...e.com>
> ---
>
> b/arch/x86/include/asm/cpuid.h | 59 ++++++++++++++++++++++++++++++++++++++
> b/arch/x86/kernel/cpu/common.c | 13 +++-----
> b/arch/x86/kernel/cpu/transmeta.c | 9 +----
> b/arch/x86/xen/enlighten_pv.c | 9 +----
> 4 files changed, 69 insertions(+), 21 deletions(-)
>
> diff -puN arch/x86/include/asm/cpuid.h~cpuid-regions arch/x86/include/asm/cpuid.h
> --- a/arch/x86/include/asm/cpuid.h~cpuid-regions 2024-03-18 15:12:20.676308753 -0700
> +++ b/arch/x86/include/asm/cpuid.h 2024-03-22 09:17:13.296507986 -0700
> @@ -168,4 +168,63 @@ static inline uint32_t hypervisor_cpuid_
> return 0;
> }
>
> +/*
> + * By convention, CPUID is broken up into regions which each
> + * have 2^16 leaves. EAX in the first leaf of each valid
> + * region returns the maximum valid leaf in that region.
> + *
> + * The regions can be thought of as being vendor-specific
> + * areas of CPUID, but that's imprecise because everybody
> + * implements the "Intel" region and Intel implements the
> + * AMD region. There are a few well-known regions:
> + * - Intel (0x0000)
> + * - AMD (0x8000)
> + * - Transmeta (0x8086)
> + * - Centaur (0xC000)
> + *
> + * Consider a CPU that where the maximum leaf in the Transmeta
> + * region is 2. On such a CPU, leaf 0x80860000 would contain:
> + * EAX==0x80860002.
> + * region-^^^^
> + * max leaf-^^^^
Minor nit:
s/a CPU that where the
/a CPU where the
> + * possible for the last basic leaf to _resemble_ a
> + * valid first leaf from a region that doesn't exist.
> + * But Intel at least seems to pad out the basic region
> + * with 0's, possibly to avoid this.
> + */
> + if ((eax >> 16) != region)
> + return 0;
> +
> + return eax;
There's whitespace damage at the 'if' line.
Thanks,
Ingo
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