[PATCH v2] arm64/sve: Don't zero the SVE state buffer when the SVE state is live

[Breno Leitao]

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.

It is worth skipping the zeroing 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 at debian.org>
---
Changes in v2:
- Rewrote the commit message using Mark's suggested wording: what
  matters is that the in-memory copy is stale whenever the state is
  live, not the format it was last saved in
- Link to v1:
  https://patch.msgid.link/20260914-b4-arm64-sve-acc-memset-v1-1-67866e442393@debian.org
---
 arch/arm64/kernel/fpsimd.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

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);

---
base-commit: f2bfbc3554ca6919484030729424b9dee2942d24
change-id: 20260911-b4-arm64-sve-acc-memset-3425ad567857

Best regards,
--  
Breno Leitao <leitao at debian.org>