Re: [Patch v5 06/19] perf/x86: Add support for XMM registers in non-PEBS and REGS_USER

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Dec 03, 2025 at 02:54:47PM +0800, Dapeng Mi wrote:
> From: Kan Liang <kan.liang@linux.intel.com>
> 
> While collecting XMM registers in a PEBS record has been supported since
> Icelake, non-PEBS events have lacked this feature. By leveraging the
> xsaves instruction, it is now possible to snapshot XMM registers for
> non-PEBS events, completing the feature set.
> 
> To utilize the xsaves instruction, a 64-byte aligned buffer is required.
> A per-CPU ext_regs_buf is added to store SIMD and other registers, with
> the buffer size being approximately 2K. The buffer is allocated using
> kzalloc_node(), ensuring natural alignment and 64-byte alignment for all
> kmalloc() allocations with powers of 2.
> 
> The XMM sampling support is extended for both REGS_USER and REGS_INTR.
> For REGS_USER, perf_get_regs_user() returns the registers from
> task_pt_regs(current), which is a pt_regs structure. It needs to be
> copied to user space secific x86_user_regs structure since kernel may
> modify pt_regs structure later.
> 
> For PEBS, XMM registers are retrieved from PEBS records.
> 
> In cases where userspace tasks are trapped within kernel mode (e.g.,
> during a syscall) when an NMI arrives, pt_regs information can still be
> retrieved from task_pt_regs(). However, capturing SIMD and other
> xsave-based registers in this scenario is challenging. Therefore,
> snapshots for these registers are omitted in such cases.
> 
> The reasons are:
> - Profiling a userspace task that requires SIMD/eGPR registers typically
>   involves NMIs hitting userspace, not kernel mode.
> - Although it is possible to retrieve values when the TIF_NEED_FPU_LOAD
>   flag is set, the complexity introduced to handle this uncommon case in
>   the critical path is not justified.
> - Additionally, checking the TIF_NEED_FPU_LOAD flag alone is insufficient.
>   Some corner cases, such as an NMI occurring just after the flag switches
>   but still in kernel mode, cannot be handled.

Urgh.. Dave, Thomas, is there any reason we could not set
TIF_NEED_FPU_LOAD *after* doing the XSAVE (clearing is already done
after restore).

That way, when an NMI sees TIF_NEED_FPU_LOAD it knows the task copy is
consistent.

I'm not at all sure this is complex, it just needs a little care.

And then there is the deferred thing, just like unwind, we can defer
REGS_USER/STACK_USER much the same, except someone went and built all
that deferred stuff with unwind all tangled into it :/