arm64/sve: Support vector length resetting for new processes

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
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>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

3 files changed, 14 insertions, 4 deletions

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index e2f575dbdddd..c6fddb005dc2 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -107,6 +107,7 @@ 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 92b7b48576c8..eb431286bacd 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -95,6 +95,7 @@ void arch_release_task_struct(struct task_struct *tsk);
 #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 000b5f9215c6..c7531b85b303 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -111,6 +111,9 @@
  */
 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;
+
 /*
  * Call __sve_free() directly only if you know task can't be scheduled
  * or preempted.
@@ -474,15 +477,20 @@ 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);