Re: [PATCH v2 3/3] arm64/sve: Skip flushing Z registers with 128 bit vectors

When the SVE vector length is 128 bits then there are no bits in the Z
registers which are not shared with the V registers so we can skip them
when zeroing state not shared with FPSIMD, this results in a minor
performance improvement.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/include/asm/fpsimd.h  | 2 +-
 arch/arm64/kernel/entry-fpsimd.S | 9 +++++++--
 arch/arm64/kernel/fpsimd.c       | 6 ++++--
 3 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
index 2599504674b5..c072161d5c65 100644
--- a/arch/arm64/include/asm/fpsimd.h
+++ b/arch/arm64/include/asm/fpsimd.h

@@ -69,7 +69,7 @@ static inline void *sve_pffr(struct thread_struct *thread)
 extern void sve_save_state(void *state, u32 *pfpsr);
 extern void sve_load_state(void const *state, u32 const *pfpsr,
 			   unsigned long vq_minus_1);
-extern void sve_flush_live(void);
+extern void sve_flush_live(unsigned long vq_minus_1);
 extern void sve_load_from_fpsimd_state(struct user_fpsimd_state const *state,
 				       unsigned long vq_minus_1);
 extern unsigned int sve_get_vl(void);

diff --git a/arch/arm64/kernel/entry-fpsimd.S b/arch/arm64/kernel/entry-fpsimd.S
index dd8382e5ce82..87ef25836963 100644
--- a/arch/arm64/kernel/entry-fpsimd.S
+++ b/arch/arm64/kernel/entry-fpsimd.S

@@ -69,10 +69,15 @@ SYM_FUNC_START(sve_load_from_fpsimd_state)
 	ret
 SYM_FUNC_END(sve_load_from_fpsimd_state)
 
-/* Zero all SVE registers but the first 128-bits of each vector */
+/*
+ * Zero all SVE registers but the first 128-bits of each vector
+ *
+ * x0 = VQ - 1

+ */
 SYM_FUNC_START(sve_flush_live)
+	cbz		x0, 1f	// A VQ-1 of 0 is 128 bits so no extra Z state
 	sve_flush_z
-	sve_flush_p_ffr
+1:	sve_flush_p_ffr
 	ret
 SYM_FUNC_END(sve_flush_live)
 

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index ad3dd34a83cf..e57b23f95284 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c

@@ -957,8 +957,10 @@ void do_sve_acc(unsigned int esr, struct pt_regs *regs)
 	 * disabling the trap, otherwise update our in-memory copy.
 	 */
 	if (!test_thread_flag(TIF_FOREIGN_FPSTATE)) {
-		sve_set_vq(sve_vq_from_vl(current->thread.sve_vl) - 1);
-		sve_flush_live();
+		unsigned long vq_minus_one =
+			sve_vq_from_vl(current->thread.sve_vl) - 1;
+		sve_set_vq(vq_minus_one);
+		sve_flush_live(vq_minus_one);
 		fpsimd_bind_task_to_cpu();
 	} else {
 		fpsimd_to_sve(current);

-- 
2.20.1