arm64/sve: Add prctl controls for userspace vector length management

This patch adds two arm64-specific prctls, to permit userspace to
control its vector length:

* PR_SVE_SET_VL: set the thread's SVE vector length and vector
length inheritance mode.

* PR_SVE_GET_VL: get the same information.

Although these prctls resemble instruction set features in the SVE
architecture, they provide additional control: the vector length
inheritance mode is Linux-specific and nothing to do with the
architecture, and the architecture does not permit EL0 to set its
own vector length directly. Both can be used in portable tools
without requiring the use of SVE instructions.

Signed-off-by: Dave Martin <[e-mail]>
Reviewed-by: Catalin Marinas <[e-mail]>
Cc: Alex Bennée <[e-mail]>
[will: Fixed up prctl constants to avoid clash with PDEATHSIG]
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/include/asm/fpsimd.h

···
 #define __ASM_FP_H
 
 #include <asm/ptrace.h>
+#include <asm/errno.h>
 
 #ifndef __ASSEMBLY__
 
···
 extern int sve_set_vector_length(struct task_struct *task,
 				 unsigned long vl, unsigned long flags);
 
+extern int sve_set_current_vl(unsigned long arg);
+extern int sve_get_current_vl(void);
+
 /*
  * Probing and setup functions.
  * Calls to these functions must be serialised with one another.
···
 static inline void fpsimd_release_task(struct task_struct *task) { }
 static inline void sve_sync_to_fpsimd(struct task_struct *task) { }
 static inline void sve_sync_from_fpsimd_zeropad(struct task_struct *task) { }
+
+static inline int sve_set_current_vl(unsigned long arg)
+{
+	return -EINVAL;
+}
+
+static inline int sve_get_current_vl(void)
+{
+	return -EINVAL;
+}
 
 static inline void sve_init_vq_map(void) { }
 static inline void sve_update_vq_map(void) { }

arch/arm64/include/asm/processor.h

···
 int cpu_enable_pan(void *__unused);
 int cpu_enable_cache_maint_trap(void *__unused);
 
+/* Userspace interface for PR_SVE_{SET,GET}_VL prctl()s: */
+#define SVE_SET_VL(arg)	sve_set_current_vl(arg)
+#define SVE_GET_VL()	sve_get_current_vl()
+
 #endif /* __ASSEMBLY__ */
 #endif /* __ASM_PROCESSOR_H */

arch/arm64/kernel/fpsimd.c

···
 #include <linux/irqflags.h>
 #include <linux/init.h>
 #include <linux/percpu.h>
+#include <linux/prctl.h>
 #include <linux/preempt.h>
 #include <linux/prctl.h>
 #include <linux/ptrace.h>
···
 		clear_tsk_thread_flag(task, TIF_SVE_VL_INHERIT);
 
 	return 0;
+}
+
+/*
+ * Encode the current vector length and flags for return.
+ * This is only required for prctl(): ptrace has separate fields
+ *
+ * flags are as for sve_set_vector_length().
+ */
+static int sve_prctl_status(unsigned long flags)
+{
+	int ret;
+
+	if (flags & PR_SVE_SET_VL_ONEXEC)
+		ret = current->thread.sve_vl_onexec;
+	else
+		ret = current->thread.sve_vl;
+
+	if (test_thread_flag(TIF_SVE_VL_INHERIT))
+		ret |= PR_SVE_VL_INHERIT;
+
+	return ret;
+}
+
+/* PR_SVE_SET_VL */
+int sve_set_current_vl(unsigned long arg)
+{
+	unsigned long vl, flags;
+	int ret;
+
+	vl = arg & PR_SVE_VL_LEN_MASK;
+	flags = arg & ~vl;
+
+	if (!system_supports_sve())
+		return -EINVAL;
+
+	ret = sve_set_vector_length(current, vl, flags);
+	if (ret)
+		return ret;
+
+	return sve_prctl_status(flags);
+}
+
+/* PR_SVE_GET_VL */
+int sve_get_current_vl(void)
+{
+	if (!system_supports_sve())
+		return -EINVAL;
+
+	return sve_prctl_status(0);
 }
 
 /*

include/uapi/linux/prctl.h

···
 # define PR_CAP_AMBIENT_CLEAR_ALL	4
 
 /* arm64 Scalable Vector Extension controls */
+/* Flag values must be kept in sync with ptrace NT_ARM_SVE interface */
+#define PR_SVE_SET_VL			50	/* set task vector length */
 # define PR_SVE_SET_VL_ONEXEC		(1 << 18) /* defer effect until exec */
+#define PR_SVE_GET_VL			51	/* get task vector length */
+/* Bits common to PR_SVE_SET_VL and PR_SVE_GET_VL */
 # define PR_SVE_VL_LEN_MASK		0xffff
 # define PR_SVE_VL_INHERIT		(1 << 17) /* inherit across exec */
 

kernel/sys.c

···
 #ifndef SET_FP_MODE
 # define SET_FP_MODE(a,b)	(-EINVAL)
 #endif
+#ifndef SVE_SET_VL
+# define SVE_SET_VL(a)		(-EINVAL)
+#endif
+#ifndef SVE_GET_VL
+# define SVE_GET_VL()		(-EINVAL)
+#endif
 
 /*
  * this is where the system-wide overflow UID and GID are defined, for
···
 		break;
 	case PR_GET_FP_MODE:
 		error = GET_FP_MODE(me);
+		break;
+	case PR_SVE_SET_VL:
+		error = SVE_SET_VL(arg2);
+		break;
+	case PR_SVE_GET_VL:
+		error = SVE_GET_VL();
 		break;
 	default:
 		error = -EINVAL;