[PATCH v6] arm64/fpsimd: Suppress SVE access traps when loading FPSIMD state

[Mark Brown]

When we are in a syscall we take the opportunity to discard the SVE state,
saving only the FPSIMD subset of the register state. When we reload the
state from memory we reenable SVE access traps, stopping tracking SVE until
the task takes another SVE access trap. This means that for a task which is
actively using SVE many blocking system calls will have the additional
overhead of a SVE access trap.

As SVE deployment is progressing we are seeing much wider use of the SVE
instruction set, including performance optimised implementations of
operations like memset() and memcpy(), which mean that even tasks which are
not obviously floating point based can end up with substantial SVE usage.

It does not, however, make sense to just unconditionally use the full SVE
register state all the time since it is larger than the FPSIMD register
state so there is overhead saving and restoring it on context switch and
our requirement to flush the register state not shared with FPSIMD on
syscall also creates a noticeable overhead on system call.

I did some instrumentation which counted the number of SVE access traps
and the number of times we loaded FPSIMD only register state for each task.
Testing with Debian Bookworm this showed that during boot the overwhelming
majority of tasks triggered another SVE access trap more than 50% of the
time after loading FPSIMD only state with a substantial number near 100%,
though some programs had a very small number of SVE accesses most likely
from startup. There were few tasks in the range 5-45%, most tasks either
used SVE frequently or used it only a tiny proportion of times. As expected
older distributions which do not have the SVE performance work available
showed no SVE usage in general applications.

This indicates that there should be some useful benefit from reducing the
number of SVE access traps for blocking system calls like we did for non
blocking system calls in commit 8c845e273104 ("arm64/sve: Leave SVE enabled
on syscall if we don't context switch"). Let's do this with a timeout, when
we take a SVE access trap record a jiffies after which we'll reeanble SVE
traps then check this whenver we load a FPSIMD only floating point state
from memory. If the time has passed then we reenable traps, otherwise we
leave traps disabled and flush the non-shared register state like we would
on trap.

The timeout is currently set to a second, I pulled this number out of thin
air so there is doubtless some room for tuning. This means that for a
task which is actively using SVE the number of SVE access traps will be
substantially reduced but applications which use SVE only very
infrequently will avoid the overheads associated with tracking SVE state
after a second. The extra cost from additional tracking of SVE state
only occurs when a task is preempted so short running tasks should be
minimally affected.

There should be no functional change resulting from this, it is purely a
performance optimisation.

Signed-off-by: Mark Brown <broonie at kernel.org>
---
Changes in v6:
- Rebase onto v6.10-rc1.
- Link to v5: https://lore.kernel.org/r/20240405-arm64-sve-trap-mitigation-v5-1-126fe2515ef1@kernel.org

Changes in v5:
- Rebase onto v6.9-rc1.
- Use a timeout rather than number of state loads to decide when to
  reenable traps.
- Link to v4: https://lore.kernel.org/r/20240122-arm64-sve-trap-mitigation-v4-1-54e0d78a3ae9@kernel.org

Changes in v4:
- Rebase onto v6.8-rc1.
- Link to v3: https://lore.kernel.org/r/20231113-arm64-sve-trap-mitigation-v3-1-4779c9382483@kernel.org

Changes in v3:
- Rebase onto v6.7-rc1.
- Link to v2: https://lore.kernel.org/r/20230913-arm64-sve-trap-mitigation-v2-1-1bdeff382171@kernel.org

Changes in v2:
- Rebase onto v6.6-rc1.
- Link to v1: https://lore.kernel.org/r/20230807-arm64-sve-trap-mitigation-v1-1-d92eed1d2855@kernel.org
---
 arch/arm64/include/asm/processor.h |  1 +
 arch/arm64/kernel/fpsimd.c         | 42 ++++++++++++++++++++++++++++++++------
 2 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index f77371232d8c..7a6ed0551291 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -164,6 +164,7 @@ struct thread_struct {
 	unsigned int		fpsimd_cpu;
 	void			*sve_state;	/* SVE registers, if any */
 	void			*sme_state;	/* ZA and ZT state, if any */
+	unsigned long		sve_timeout;    /* jiffies to drop TIF_SVE */
 	unsigned int		vl[ARM64_VEC_MAX];	/* vector length */
 	unsigned int		vl_onexec[ARM64_VEC_MAX]; /* vl after next exec */
 	unsigned long		fault_address;	/* fault info */
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 82e8a6017382..4741e4fb612a 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -354,6 +354,7 @@ static void task_fpsimd_load(void)
 {
 	bool restore_sve_regs = false;
 	bool restore_ffr;
+	unsigned long sve_vq_minus_one;
 
 	WARN_ON(!system_supports_fpsimd());
 	WARN_ON(preemptible());
@@ -365,18 +366,12 @@ static void task_fpsimd_load(void)
 	if (system_supports_sve() || system_supports_sme()) {
 		switch (current->thread.fp_type) {
 		case FP_STATE_FPSIMD:
-			/* Stop tracking SVE for this task until next use. */
-			if (test_and_clear_thread_flag(TIF_SVE))
-				sve_user_disable();
 			break;
 		case FP_STATE_SVE:
 			if (!thread_sm_enabled(&current->thread) &&
 			    !WARN_ON_ONCE(!test_and_set_thread_flag(TIF_SVE)))
 				sve_user_enable();
 
-			if (test_thread_flag(TIF_SVE))
-				sve_set_vq(sve_vq_from_vl(task_get_sve_vl(current)) - 1);
-
 			restore_sve_regs = true;
 			restore_ffr = true;
 			break;
@@ -395,6 +390,15 @@ static void task_fpsimd_load(void)
 		}
 	}
 
+	/*
+	 * If SVE has been enabled we may keep it enabled even if
+	 * loading only FPSIMD state, so always set the VL.
+	 */
+	if (system_supports_sve() && test_thread_flag(TIF_SVE)) {
+		sve_vq_minus_one = sve_vq_from_vl(task_get_sve_vl(current)) - 1;
+		sve_set_vq(sve_vq_minus_one);
+	}
+
 	/* Restore SME, override SVE register configuration if needed */
 	if (system_supports_sme()) {
 		unsigned long sme_vl = task_get_sme_vl(current);
@@ -421,6 +425,25 @@ static void task_fpsimd_load(void)
 	} else {
 		WARN_ON_ONCE(current->thread.fp_type != FP_STATE_FPSIMD);
 		fpsimd_load_state(&current->thread.uw.fpsimd_state);
+
+		/*
+		 * If the task had been using SVE we keep it enabled
+		 * when loading FPSIMD only state for a period to
+		 * minimise overhead for tasks actively using SVE,
+		 * disabling it periodicaly to ensure that tasks that
+		 * use SVE intermittently do eventually avoid the
+		 * overhead of carrying SVE state.  The timeout is
+		 * initialised when we take a SVE trap in in
+		 * do_sve_acc().
+		 */
+		if (system_supports_sve() && test_thread_flag(TIF_SVE)) {
+			if (time_after(jiffies, current->thread.sve_timeout)) {
+				clear_thread_flag(TIF_SVE);
+				sve_user_disable();
+			} else {
+				sve_flush_live(true, sve_vq_minus_one);
+			}
+		}
 	}
 }
 
@@ -1397,6 +1420,13 @@ void do_sve_acc(unsigned long esr, struct pt_regs *regs)
 
 	get_cpu_fpsimd_context();
 
+	/*
+	 * We will keep SVE enabled when loading FPSIMD only state for
+	 * the next second to minimise traps when userspace is
+	 * actively using SVE.
+	 */
+	current->thread.sve_timeout = jiffies + HZ;
+
 	if (test_and_set_thread_flag(TIF_SVE))
 		WARN_ON(1); /* SVE access shouldn't have trapped */
 

---
base-commit: 1613e604df0cd359cf2a7fbd9be7a0bcfacfabd0
change-id: 20230807-arm64-sve-trap-mitigation-2e7e2663c849

Best regards,
-- 
Mark Brown <broonie at kernel.org>