linux-kernel - Re: [PATCH 01/74] x86/cpu/vfm: Add/initialize x86_vfm field to struct cpuinfo

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Date: Fri, 29 Mar 2024 12:40:07 +0100
From: Borislav Petkov <bp@...en8.de>
To: "Luck, Tony" <tony.luck@...el.com>
Cc: "x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/74] x86/cpu/vfm: Add/initialize x86_vfm field to
 struct cpuinfo_x86

On Thu, Mar 28, 2024 at 06:32:35PM +0000, Luck, Tony wrote:
> I don't think the format is really that big an issue. Including stepping in the
> format adds complexity to a thousand places these checks are made while
> only being useful in a few dozen.

I've figured out what the problem is with steppings - ranges. If you
have a range of steppings which all belong to the same model, then you
have to complicate the checks by either masking out the stepping or use
the X86_STEPPING_ANY thing which forces you to use x86_match_cpu()
instead of a simple integer comparison.

And you should talk to your folks what their plan is for the new
families because if they do a range of model numbers which all belong to
the same CPU model like AMD does, then your simple comparison scheme
goes out the window because it can't really deal with ranges.

Because from looking at your set, I don't see a slick way to check
whether a concrete f/m/s tuple belongs to a range without involved
checking.

For example, models:

                case 0x30 ... 0x4f:
                case 0x60 ... 0x7f:
                case 0x90 ... 0x91:
                case 0xa0 ... 0xaf:

are all Zen2. I could do a X86_MATCH_VF_MODEL_RANGE and we even had
a patch like that at some point but it didn't go in. But even if I did
that, I'd still need to do x86_match_cpu() instead of the current
X86_FEATURE_ZEN* checks we're doing.

So I don't think I can switch AMD to use that. It looks like the 'V' in
"VFM" could just as well turn into "I".

:-)

I'd say.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette