linux-kernel - Re: [PATCH V3 05/17] perf/x86: Support XMM register for non-PEBS and REGS

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Message-ID: <dbd28326-1d47-462e-947c-34e3c4a08ec6@linux.intel.com>
Date: Thu, 21 Aug 2025 09:00:45 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: "Liang, Kan" <kan.liang@...ux.intel.com>,
 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,
 ak@...ux.intel.com, zide.chen@...el.com, mark.rutland@....com,
 broonie@...nel.org, ravi.bangoria@....com, eranian@...gle.com
Subject: Re: [PATCH V3 05/17] perf/x86: Support XMM register for non-PEBS and
 REGS_USER


On 8/21/2025 2:03 AM, Liang, Kan wrote:
>
> On 2025-08-20 2:46 a.m., Mi, Dapeng wrote:
>> On 8/19/2025 11:55 PM, Liang, Kan wrote:
>>> On 2025-08-19 6:39 a.m., Peter Zijlstra wrote:
>>>> On Fri, Aug 15, 2025 at 02:34:23PM -0700, kan.liang@...ux.intel.com wrote:
>>>>> From: Kan Liang <kan.liang@...ux.intel.com>
>>>>>
>>>>> Collecting the XMM registers in a PEBS record has been supported since
>>>>> the Icelake. But non-PEBS events don't support the feature. It's
>>>>> possible to retrieve the XMM registers from the XSAVE for non-PEBS.
>>>>> Add it to make the feature complete.
>>>>>
>>>>> To utilize the XSAVE, a 64-byte aligned buffer is required. Add a
>>>>> per-CPU ext_regs_buf to store the vector registers. The size of the
>>>>> buffer is ~2K. kzalloc_node() is used because there's a _guarantee_
>>>>> that all kmalloc()'s with powers of 2 are naturally aligned and also
>>>>> 64b aligned.
>>>>>
>>>>> Extend the support for both REGS_USER and REGS_INTR. For REGS_USER, the
>>>>> perf_get_regs_user() returns the regs from the task_pt_regs(current),
>>>>> which is struct pt_regs. Need to move it to local struct x86_perf_regs
>>>>> x86_user_regs.
>>>>> For PEBS, the HW support is still preferred. The XMM should be retrieved
>>>>> from PEBS records.
>>>>>
>>>>> There could be more vector registers supported later. Add ext_regs_mask
>>>>> to track the supported vector register group.
>>>> I'm a little confused... *again* :-)
>>>>
>>>> Specifically, we should consider two sets of registers:
>>>>
>>>>  - the live set, as per the CPU (XSAVE)
>>>>  - the stored set, as per x86_task_fpu()
>>>>
>>>> regs_intr should always get a copy of the live set; however
>>>> regs_user should not. It might need a copy of the x86_task_fpu() instead
>>>> of the live set, depending on TIF_NEED_FPU_LOAD (more or less, we need
>>>> another variable set in kernel_fpu_begin_mask() *after*
>>>> save_fpregs_to_fpstate() is completed).
>>>>
>>>> I don't see this code make this distinction.
>>>>
>>>> Consider getting a sample while the kernel is doing some avx enhanced
>>>> crypto and such.
>>> The regs_user only needs a set when the NMI hits the user mode
>>> (user_mode(regs)) or a non-kernel thread (!(current->flags &
>>> PF_KTHREAD)). The live set is good enough for both cases.
>> It's fine if NMI hits user mode, but if NMI hits the kernel mode
>> (!(current->flags &PF_KTHREAD)), won't the kernel space SIMD/eGPR regs be
>> exposed to user space for user-regs option? I'm not sure if kernel space
>> really use these SIMD/eGPR regs right now, but it seems a risk.
>>
>>
> I don't think it's possible for the existing kernel. But I cannot
> guarantee future usage.
>
> If the kernel mode handling is still a concern, I think we should drop
> the SIMD/eGPR regs for the case for now. Because
> - To profile a userspace application which requires SIMD/eGPR regs, the
> NMI usually hits the usersapce. It's not common to hit the kernel mode.
> - The SIMD/eGPR cannot be retrieved from the task_pt_regs(). Although
> it's possible to retrieve the values when the TIF_NEED_FPU_LOAD flag is
> set, I don't think it's worth introducing such complexity to handle an
> uncommon case in the critical path.
> - Furthermore, only checking the TIF_NEED_FPU_LOAD flag cannot cure
> everything. Some corner cases cannot be handled either. For example, an
> NMI can happen when the flag just switched, but still in the kernel mode.
>
> We can always add the support later if someone thinks it's important to
> retrieve the user SIMD/eGPR regs during the kernel syscall.

+1


>
> Thanks,
> Kan
>>> I think the kernel crypto should be to a kernel thread (current->flags &
>>> PF_KTHREAD). If so, the regs_user should return NULL.
>>>
>>> Thanks,
>>> Kan
>>>