linux-kernel - Re: [RFC PATCH 00/12] Support vector and more extended registers in perf

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Message-ID: <893f1888-b8cb-4976-a0d6-606460438d73@linux.intel.com>
Date: Tue, 17 Jun 2025 11:23:10 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...hat.com, acme@...nel.org, namhyung@...nel.org,
 tglx@...utronix.de, dave.hansen@...ux.intel.com, irogers@...gle.com,
 adrian.hunter@...el.com, jolsa@...nel.org,
 alexander.shishkin@...ux.intel.com, linux-kernel@...r.kernel.org,
 dapeng1.mi@...ux.intel.com, ak@...ux.intel.com, zide.chen@...el.com
Subject: Re: [RFC PATCH 00/12] Support vector and more extended registers in
 perf



On 2025-06-17 10:29 a.m., Peter Zijlstra wrote:
> On Tue, Jun 17, 2025 at 09:52:12AM -0400, Liang, Kan wrote:
> 
>> OK. So the sample_simd_reg_words actually has another meaning now.
> 
> Well, any simd field being non-zero means userspace knows about it. Sort
> of an implicit flag.

Yes, but the tool probably wouldn't to touch any simd fields if user
doesn't ask for simd registers

> 
>> It's used as a flag to tell whether utilizing the old format.
>>
>> If so, I think it may be better to have a dedicate sample_simd_reg_flag
>> field.
>>
>> For example,
>>
>> #define SAMPLE_SIMD_FLAGS_FORMAT_LEGACY		0x0
>> #define SAMPLE_SIMD_FLAGS_FORMAT_WORDS		0x1
>>
>> 	__u8 sample_simd_reg_flags;
>> 	__u8 sample_simd_reg_words;
>> 	__u64 sample_simd_reg_intr;
>> 	__u64 sample_simd_reg_user;
>>
>> If (sample_simd_reg_flags != 0) reclaims the XMM space for APX and SPP.
>>
>> Does it make sense?
> 
> Not sure, it eats up a whole byte. Dapeng seemed to favour separate
> intr/user vector width (although I'm not quite sure what the use would
> be).
> 
> If you want an explicit bit, we might as well use one from __reserved_1,
> we still have some left.

OK. I may add a sample_simd_reg : 1 to explicitly tell kernel to utilize
the sample_simd_reg_XXX.

Thanks,
Kan