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.

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@debian.org>
Acked-by: Mark Rutlame <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

0 files changed, 0 insertions, 0 deletions