[PATCH v8 2/2] arm64/sve: Disable TIF_SVE on syscall once per second

[Mark Brown <broonie@kernel.org>]

Our syscall ABI requires that when performing a syscall the portions of
the Z registers not shared with the V registers, the P and FFR registers
are reset to 0.  Since we have no way of monitoring EL0 SVE usage this
is implemented by changing the in register values on every syscall for
tasks which have SVE enabled, for systems with 128 bit SVE vector
lengths this has been benchmarked as a 6% overhead.

We currently support disabling SVE for userspace tasks when loading the
floating point state from memory during a syscall, allowing tasks that
use SVE infrequently to avoid this overhead, but this may not help CPU
bound tasks if they are not fortunate enough to block or be scheduled
during a syscall.  This is done whenever the state is loaded from a
second after the last time the task generate a SVE access trap.

Extend this mechanism to also apply during syscall entry, disabling SVE
instead of flushing the live registers when we perform a syscall a
second after the last time a SVE access trap was taken.  This adds an
additional memory access and branch for tasks using SVE and means that
CPU bound tasks actively using SVE will take extra SVE access traps (at
most one per second) but will allows CPU bound tasks that infrequently
use SVE to avoid the overhead of flushing the registers on syscall.

On a system with 128 bit SVE vectors fp-pidbench shows a roughly 4.5%
improvement compared to baseline after having used SVE, for a roughly
0.4% overhead when SVE is used between each syscall.  Obviously this is
very much a microbenchmark.

This is purely a performance optimisation, there should be no functional
change.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/kernel/entry-common.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index 3625797e9ee8..a7b7ec66f084 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -234,8 +234,18 @@ static inline void fpsimd_syscall_enter(void)
 	if (test_thread_flag(TIF_SVE)) {
 		unsigned int sve_vq_minus_one;
 
-		sve_vq_minus_one = sve_vq_from_vl(task_get_sve_vl(current)) - 1;
-		sve_flush_live(true, sve_vq_minus_one);
+		/*
+		 * Ensure that tasks that don't block in a syscall
+		 * also get a chance to drop TIF_SVE.
+		 */
+		if (unlikely(time_after(jiffies,
+					current->thread.sve_timeout))) {
+			clear_thread_flag(TIF_SVE);
+			sve_user_disable();
+		} else {
+			sve_vq_minus_one = sve_vq_from_vl(task_get_sve_vl(current)) - 1;
+			sve_flush_live(true, sve_vq_minus_one);
+		}
 	}
 
 	/*

-- 
2.47.3