[PATCH v2 12/28] arm64/sve: Support vector length resetting for new processes

[Dave Martin <Dave dot Martin at arm dot 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>
Cc: Alex Bennée <alex.bennee@linaro.org>

---

Changes since v1
----------------

Requested by Alex Bennée:

* Make sve_vl_onexec an unsigned int: that's the type used virtually
everywhere else, and the memory saving is too minor to be interesting.
---
 arch/arm64/include/asm/processor.h   |  1 +
 arch/arm64/include/asm/thread_info.h |  1 +
 arch/arm64/kernel/fpsimd.c           | 16 ++++++++++++----
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 4831d28..3faceac 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 */
 	unsigned int		sve_vl;		/* SVE vector length */
+	unsigned int		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 f0880fc..d3568ab 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -92,6 +92,7 @@ void arch_setup_new_exec(void);
 #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 9b1ebd7..e20b44d 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -106,6 +106,9 @@
  */
 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 = SVE_VL_MIN;
+
 static void sve_free(struct task_struct *task)
 {
 	kfree(task->thread.sve_state);
@@ -380,15 +383,20 @@ void fpsimd_flush_thread(void)
 		 * If a bug causes this to go wrong, we make some noise and
 		 * try to fudge thread.sve_vl to a safe value here.
 		 */
-		vl = current->thread.sve_vl;
-
-		if (vl == 0)
-			vl = SVE_VL_MIN;
+		vl = current->thread.sve_vl_onexec ?
+			current->thread.sve_vl_onexec : sve_default_vl;
 
 		if (WARN_ON(!sve_vl_valid(vl)))
 			vl = SVE_VL_MIN;
 
 		current->thread.sve_vl = 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