[PATCH 12/27] arm64/sve: Support vector length resetting for new processes

[Dave Martin <Dave.Martin@arm.com>]

It's desirable to be able to reset the vector length to some sane
default for new processes, since the new binary and its libraries
processes may or may not be SVE-aware.

This patch tracks the desired post-exec vector length (if any) in a
new thread member sve_vl_onexec, and adds a new thread flag
TIF_SVE_VL_INHERIT to control whether to inherit or reset the
vector length.  Currently these are inactive.  Subsequent patches
will provide the capability to configure them.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
---
 arch/arm64/include/asm/processor.h   |  1 +
 arch/arm64/include/asm/thread_info.h |  1 +
 arch/arm64/kernel/fpsimd.c           | 13 +++++++++++++
 3 files changed, 15 insertions(+)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 969feed..da8802a 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -87,6 +87,7 @@ struct thread_struct {
 	struct fpsimd_state	fpsimd_state;
 	void			*sve_state;	/* SVE registers, if any */
 	u16			sve_vl;		/* SVE vector length */
+	u16			sve_vl_onexec;	/* SVE vl after next exec */
 	unsigned long		fault_address;	/* fault info */
 	unsigned long		fault_code;	/* ESR_EL1 value */
 	struct debug_info	debug;		/* debugging */
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index 1a4b30b..bf9c552 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -97,6 +97,7 @@ struct thread_info {
 #define TIF_SINGLESTEP		21
 #define TIF_32BIT		22	/* 32bit process */
 #define TIF_SVE			23	/* Scalable Vector Extension in use */
+#define TIF_SVE_VL_INHERIT	24	/* Inherit sve_vl_onexec across exec */
 
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 37dd1b2..80ecb2d 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -108,6 +108,9 @@ static void task_fpsimd_save(void);
  */
 static DEFINE_PER_CPU(struct fpsimd_state *, fpsimd_last_state);
 
+/* Default VL for tasks that don't set it explicitly: */
+static int sve_default_vl = -1;
+
 static void sve_free(struct task_struct *task)
 {
 	kfree(task->thread.sve_state);
@@ -392,6 +395,9 @@ void fpsimd_flush_thread(void)
 		clear_thread_flag(TIF_SVE);
 		sve_free(current);
 
+		current->thread.sve_vl = current->thread.sve_vl_onexec ?
+			current->thread.sve_vl_onexec : sve_default_vl;
+
 		/*
 		 * User tasks must have a valid vector length set, but tasks
 		 * forked early (e.g., init) may not initially have one.
@@ -401,6 +407,13 @@ void fpsimd_flush_thread(void)
 		 * If not, something went badly wrong.
 		 */
 		BUG_ON(!sve_vl_valid(current->thread.sve_vl));
+
+		/*
+		 * If the task is not set to inherit, ensure that the vector
+		 * length will be reset by a subsequent exec:
+		 */
+		if (!test_thread_flag(TIF_SVE_VL_INHERIT))
+			current->thread.sve_vl_onexec = 0;
 	}
 
 	set_thread_flag(TIF_FOREIGN_FPSTATE);
-- 
2.1.4