Re: [PATCH] arm64/sve: Don't zero the SVE state buffer when the SVE state is live

[Breno Leitao <leitao@debian.org>]

Hello Mark,

On Tue, Sep 15, 2026 at 09:39:24AM +0100, Mark Rutland wrote:
> Hi Breno,
> 
> I think the change looks reasonable, but the commit message and comments
> aren't quite right. More on that below.

Thank you very much for your review. I know this is not a trivial one
(at least from my PoV), I am glad you quickly reviewed it.

I've also dropped few other lines, but kept the benchmark values I've
collected. Does this look better now?

Author: Breno Leitao <leitao@debian.org>
Date:   Fri Sep 11 02:57:34 2026 -0700

    arm64/sve: Don't zero the SVE state buffer when the SVE state is live

    Currently do_sve_acc() always zeroes current->thread.sve_state. This is
    not necessary in the common case, and avoiding the zeroing has a
    measurable impact on some benchmarks.

    In the common case where the task is not preempted and its state is not
    altered by a tracer, do_sve_acc() will observe that TIF_FOREIGN_FPSTATE
    is clear. In such cases, only the live register values matter, and the
    in-memory copy is stale regardless of whether it is saved in
    FP_STATE_FPSIMD format or FP_STATE_SVE format.

    This is worth doing because the SVE state is discarded on syscall entry,
    so userspace that mixes SVE and syscalls re-traps constantly. A fleet
    profile of arm64 hosts running services whose memset() is SVE shows the
    memset under do_sve_acc() accounting for 29% of the trap handling cost.

    Measured on a 72-core Neoverse V2 (SVE VL 128, sve_state_size 546,
    performance governor) with perf bench sched pipe pinned to one CPU, and
    SVE operation on write, so that each loop also takes an SVE access trap.

            * -0.99% kernel instructions
            * -1.38% kernel cycles
            * -1.12% wall clock

    Signed-off-by: Breno Leitao <leitao@debian.org>

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index e7f1682a3059b..324c9799b0511 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -1316,7 +1316,7 @@ void do_sve_acc(unsigned long esr, struct pt_regs *regs)
                return;
        }

-       sve_alloc(current, true);
+       sve_alloc(current, false);
        if (!current->thread.sve_state) {
                force_sig(SIGKILL);
                return;
@@ -1341,6 +1341,7 @@ void do_sve_acc(unsigned long esr, struct pt_regs *regs)
                sve_flush_live();
                fpsimd_bind_task_to_cpu();
        } else {
+               memset(current->thread.sve_state, 0, sve_state_size(current));
                fpsimd_to_sve(current);
                current->thread.fp_type = FP_STATE_SVE;
                fpsimd_flush_task_state(current);