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

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2025-06-17 8:57 p.m., Mi, Dapeng wrote:
> 
> On 6/17/2025 11:23 PM, Liang, Kan wrote:
>>
>> 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 if I missed some discussion, but are these fields only intended
> for SIMD regs? What about the APX extended GPRs? Suppose APX eGPRs can
> reuse the legacy XMM bitmaps in sample_regs_user/intr[47:32], but we need
> an extra flag to distinguish it's XMM regs or APX eGPRs, maybe add an extra
> bit sample_egpr_reg : 1 in sample_simd_reg_words, but the *simd* word in
> the name would become ambiguous.

It can be used to explicitly tell the kernel that a new format is
expected. The new format means
- Put APX and SPP into sample_regs_user/intr[47:32]
- Use the sample_simd_reg_*

Alternatively, as Peter suggested, we can use the sample_simd_reg_words
to imply the new format.
If so, I will make it an union, for example.
	union {
		__u16 sample_reg_flags;
		__u16 sample_simd_reg_words;
	};

The first thing the tool does should be to set sample_reg_flags = 1,
regardless of whether simd is requested.

> 
> 
>>> 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).
> 
> The reason that I prefer to add 2 separate "words" item is that user could
> sample interrupt and user space SIMD regs (but with different bit-width)
> simultaneously in theory, like "--intr-regs=YMM0, --user-regs=XMM0".

I'm not sure why the user wants a different bit-width. The
--user-regs=XMM0" doesn't seem to provide more useful information.

Anyway, I believe the tool can handle this case. The tool can always ask
YMM0 for both --intr-regs and --user-regs, but only output the XMM0 for
--user-regs. The only drawback is that the kernel may dump extra
information for the --user-regs. I don't think it's a big problem.

Thanks,
Kan 
> 
> 
>>>
>>> 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