[PATCH v7 06/16] arm64/sve: Refactor user SVE trap maintenance for external use

[Dave Martin]

In preparation for optimising the way KVM manages switching the
guest and host FPSIMD state, it is necessary to provide a means for
code outside arch/arm64/kernel/fpsimd.c to restore the user trap
configuration for SVE correctly for the current task.

Rather than requiring external code to duplicate the maintenance
explicitly, this patch wraps moves the trap maintenenace to
fpsimd_bind_to_cpu(), since it is logically part of the work of
associating the current task with the cpu.

Because fpsimd_bind_to_cpu() is rather a cryptic name to publish
alongside fpsimd_bind_state_to_cpu(), the former function is
renamed to fpsimd_bind_task_to_cpu() to make its purpose more
explicit.

This patch makes appropriate changes to ensure that
fpsimd_bind_task_to_cpu() is always called alongside
task_fpsimd_load(), so that the trap maintenance continues to be
done in every situation where it was done prior to this patch.

As a side-effect, the metadata updates done by
fpsimd_bind_task_to_cpu() now change from conditional to
unconditional in the "already bound" case of sigreturn.  This is
harmless, and a couple of extra stores on this slow path will not
impact performance.  I consider this a reasonable price to pay for
a slightly cleaner interface.

Signed-off-by: Dave Martin <Dave.Martin at arm.com>
---
 arch/arm64/kernel/fpsimd.c | 28 ++++++++++++++--------------
 1 file changed, 14 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 5fc0595..60404eb 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -257,16 +257,6 @@ static void task_fpsimd_load(void)
 			       sve_vq_from_vl(current->thread.sve_vl) - 1);
 	else
 		fpsimd_load_state(&current->thread.uw.fpsimd_state);
-
-	if (system_supports_sve()) {
-		/* Toggle SVE trapping for userspace if needed */
-		if (test_thread_flag(TIF_SVE))
-			sve_user_enable();
-		else
-			sve_user_disable();
-
-		/* Serialised by exception return to user */
-	}
 }
 
 /*
@@ -998,7 +988,7 @@ void fpsimd_signal_preserve_current_state(void)
  * Associate current's FPSIMD context with this cpu
  * Preemption must be disabled when calling this function.
  */
-static void fpsimd_bind_to_cpu(void)
+static void fpsimd_bind_task_to_cpu(void)
 {
 	struct fpsimd_last_state_struct *last =
 		this_cpu_ptr(&fpsimd_last_state);
@@ -1006,6 +996,16 @@ static void fpsimd_bind_to_cpu(void)
 	last->st = &current->thread.uw.fpsimd_state;
 	last->sve_in_use = test_thread_flag(TIF_SVE);
 	current->thread.fpsimd_cpu = smp_processor_id();
+
+	if (system_supports_sve()) {
+		/* Toggle SVE trapping for userspace if needed */
+		if (test_thread_flag(TIF_SVE))
+			sve_user_enable();
+		else
+			sve_user_disable();
+
+		/* Serialised by exception return to user */
+	}
 }
 
 /*
@@ -1022,7 +1022,7 @@ void fpsimd_restore_current_state(void)
 
 	if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
 		task_fpsimd_load();
-		fpsimd_bind_to_cpu();
+		fpsimd_bind_task_to_cpu();
 	}
 
 	local_bh_enable();
@@ -1045,9 +1045,9 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state)
 		fpsimd_to_sve(current);
 
 	task_fpsimd_load();
+	fpsimd_bind_task_to_cpu();
 
-	if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE))
-		fpsimd_bind_to_cpu();
+	clear_thread_flag(TIF_FOREIGN_FPSTATE);
 
 	local_bh_enable();
 }
-- 
2.1.4