Messages in this thread |  | | Date | Thu, 4 Dec 2025 16:17:35 +0100 | | From | Peter Zijlstra <> | | Subject | Re: [Patch v5 06/19] perf/x86: Add support for XMM registers in non-PEBS and REGS_USER |
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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 :/
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