arm64/sve: Core task context handling

This patch adds the core support for switching and managing the SVE
architectural state of user tasks.

Calls to the existing FPSIMD low-level save/restore functions are
factored out as new functions task_fpsimd_{save,load}(), since SVE
now dynamically may or may not need to be handled at these points
depending on the kernel configuration, hardware features discovered
at boot, and the runtime state of the task.  To make these
decisions as fast as possible, const cpucaps are used where
feasible, via the system_supports_sve() helper.

The SVE registers are only tracked for threads that have explicitly
used SVE, indicated by the new thread flag TIF_SVE.  Otherwise, the
FPSIMD view of the architectural state is stored in
thread.fpsimd_state as usual.

When in use, the SVE registers are not stored directly in
thread_struct due to their potentially large and variable size.
Because the task_struct slab allocator must be configured very
early during kernel boot, it is also tricky to configure it
correctly to match the maximum vector length provided by the
hardware, since this depends on examining secondary CPUs as well as
the primary.  Instead, a pointer sve_state in thread_struct points
to a dynamically allocated buffer containing the SVE register data,
and code is added to allocate and free this buffer at appropriate
times.

TIF_SVE is set when taking an SVE access trap from userspace, if
suitable hardware support has been detected.  This enables SVE for
the thread: a subsequent return to userspace will disable the trap
accordingly.  If such a trap is taken without sufficient system-
wide hardware support, SIGILL is sent to the thread instead as if
an undefined instruction had been executed: this may happen if
userspace tries to use SVE in a system where not all CPUs support
it for example.

The kernel will clear TIF_SVE and disable SVE for the thread
whenever an explicit syscall is made by userspace.  For backwards
compatibility reasons and conformance with the spirit of the base
AArch64 procedure call standard, the subset of the SVE register
state that aliases the FPSIMD registers is still preserved across a
syscall even if this happens.  The remainder of the SVE register
state logically becomes zero at syscall entry, though the actual
zeroing work is currently deferred until the thread next tries to
use SVE, causing another trap to the kernel.  This implementation
is suboptimal: in the future, the fastpath case may be optimised
to zero the registers in-place and leave SVE enabled for the task,
where beneficial.

TIF_SVE is also cleared in the following slowpath cases, which are
taken as reasonable hints that the task may no longer use SVE:
 * exec
 * fork and clone

Code is added to sync data between thread.fpsimd_state and
thread.sve_state whenever enabling/disabling SVE, in a manner
consistent with the SVE architectural programmer's model.
Signed-off-by: NDave Martin <Dave.Martin@arm.com>
Reviewed-by: NCatalin Marinas <catalin.marinas@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Alex Bennée <alex.bennee@linaro.org>
[will: added #include to fix allnoconfig build]
[will: use enable_daif in do_sve_acc]
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/include/asm/fpsimd.h

@@ -20,6 +20,8 @@
 
 #ifndef __ASSEMBLY__
 
+#include <linux/stddef.h>
+
 /*
  * FP/SIMD storage area has:
  *  - FPSR and FPCR
@@ -72,6 +74,20 @@ extern void sve_load_state(void const *state, u32 const *pfpsr,
 			   unsigned long vq_minus_1);
 extern unsigned int sve_get_vl(void);
 
+#ifdef CONFIG_ARM64_SVE
+
+extern size_t sve_state_size(struct task_struct const *task);
+
+extern void sve_alloc(struct task_struct *task);
+extern void fpsimd_release_task(struct task_struct *task);
+
+#else /* ! CONFIG_ARM64_SVE */
+
+static inline void sve_alloc(struct task_struct *task) { }
+static inline void fpsimd_release_task(struct task_struct *task) { }
+
+#endif /* ! CONFIG_ARM64_SVE */
+
 /* For use by EFI runtime services calls only */
 extern void __efi_fpsimd_begin(void);
 extern void __efi_fpsimd_end(void);

arch/arm64/include/asm/processor.h

@@ -105,6 +105,8 @@ struct thread_struct {
 	unsigned long		tp2_value;
 #endif
 	struct fpsimd_state	fpsimd_state;
+	void			*sve_state;	/* SVE registers, if any */
+	unsigned int		sve_vl;		/* SVE vector length */
 	unsigned long		fault_address;	/* fault info */
 	unsigned long		fault_code;	/* ESR_EL1 value */
 	struct debug_info	debug;		/* debugging */

arch/arm64/include/asm/thread_info.h

@@ -63,6 +63,8 @@ struct thread_info {
 void arch_setup_new_exec(void);
 #define arch_setup_new_exec     arch_setup_new_exec
 
+void arch_release_task_struct(struct task_struct *tsk);
+
 #endif
 
 /*
@@ -92,6 +94,7 @@ void arch_setup_new_exec(void);
 #define TIF_RESTORE_SIGMASK	20
 #define TIF_SINGLESTEP		21
 #define TIF_32BIT		22	/* 32bit process */
+#define TIF_SVE			23	/* Scalable Vector Extension in use */
 
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
@@ -105,6 +108,7 @@ void arch_setup_new_exec(void);
 #define _TIF_UPROBE		(1 << TIF_UPROBE)
 #define _TIF_FSCHECK		(1 << TIF_FSCHECK)
 #define _TIF_32BIT		(1 << TIF_32BIT)
+#define _TIF_SVE		(1 << TIF_SVE)
 
 #define _TIF_WORK_MASK		(_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
 				 _TIF_NOTIFY_RESUME | _TIF_FOREIGN_FPSTATE | \

arch/arm64/include/asm/traps.h

@@ -34,6 +34,8 @@ struct undef_hook {
 
 void register_undef_hook(struct undef_hook *hook);
 void unregister_undef_hook(struct undef_hook *hook);
+void force_signal_inject(int signal, int code, struct pt_regs *regs,
+			 unsigned long address);
 
 void arm64_notify_segfault(struct pt_regs *regs, unsigned long addr);
 

arch/arm64/kernel/entry.S

@@ -599,6 +599,8 @@ el0_sync:
 	b.eq	el0_ia
 	cmp	x24, #ESR_ELx_EC_FP_ASIMD	// FP/ASIMD access
 	b.eq	el0_fpsimd_acc
+	cmp	x24, #ESR_ELx_EC_SVE		// SVE access
+	b.eq	el0_sve_acc
 	cmp	x24, #ESR_ELx_EC_FP_EXC64	// FP/ASIMD exception
 	b.eq	el0_fpsimd_exc
 	cmp	x24, #ESR_ELx_EC_SYS64		// configurable trap
@@ -650,6 +652,7 @@ el0_svc_compat:
 	/*
 	 * AArch32 syscall handling
 	 */
+	ldr	x16, [tsk, #TSK_TI_FLAGS]	// load thread flags
 	adrp	stbl, compat_sys_call_table	// load compat syscall table pointer
 	mov	wscno, w7			// syscall number in w7 (r7)
 	mov     wsc_nr, #__NR_compat_syscalls
@@ -699,9 +702,19 @@ el0_fpsimd_acc:
 	mov	x1, sp
 	bl	do_fpsimd_acc
 	b	ret_to_user
+el0_sve_acc:
+	/*
+	 * Scalable Vector Extension access
+	 */
+	enable_daif
+	ct_user_exit
+	mov	x0, x25
+	mov	x1, sp
+	bl	do_sve_acc
+	b	ret_to_user
 el0_fpsimd_exc:
 	/*
-	 * Floating Point or Advanced SIMD exception
+	 * Floating Point, Advanced SIMD or SVE exception
 	 */
 	enable_daif
 	ct_user_exit
@@ -849,16 +862,36 @@ ENDPROC(ret_to_user)
  */
 	.align	6
 el0_svc:
+	ldr	x16, [tsk, #TSK_TI_FLAGS]	// load thread flags
 	adrp	stbl, sys_call_table		// load syscall table pointer
 	mov	wscno, w8			// syscall number in w8
 	mov	wsc_nr, #__NR_syscalls
+
+#ifndef CONFIG_ARM64_SVE
+	b	el0_svc_naked
+#else
+	tbz	x16, #TIF_SVE, el0_svc_naked	// Skip unless TIF_SVE set:
+	bic	x16, x16, #_TIF_SVE		// discard SVE state
+	str	x16, [tsk, #TSK_TI_FLAGS]
+
+	/*
+	 * task_fpsimd_load() won't be called to update CPACR_EL1 in
+	 * ret_to_user unless TIF_FOREIGN_FPSTATE is still set, which only
+	 * happens if a context switch or kernel_neon_begin() or context
+	 * modification (sigreturn, ptrace) intervenes.
+	 * So, ensure that CPACR_EL1 is already correct for the fast-path case:
+	 */
+	mrs	x9, cpacr_el1
+	bic	x9, x9, #CPACR_EL1_ZEN_EL0EN	// disable SVE for el0
+	msr	cpacr_el1, x9			// synchronised by eret to el0
+#endif /* CONFIG_ARM64_SVE */
+
 el0_svc_naked:					// compat entry point
 	stp	x0, xscno, [sp, #S_ORIG_X0]	// save the original x0 and syscall number
 	enable_daif
 	ct_user_exit 1
 
-	ldr	x16, [tsk, #TSK_TI_FLAGS]	// check for syscall hooks
-	tst	x16, #_TIF_SYSCALL_WORK
+	tst	x16, #_TIF_SYSCALL_WORK		// check for syscall hooks
 	b.ne	__sys_trace
 	cmp     wscno, wsc_nr			// check upper syscall limit
 	b.hs	ni_sys

arch/arm64/kernel/fpsimd.c

@@ -18,19 +18,28 @@
  */
 
 #include <linux/bottom_half.h>
+#include <linux/bug.h>
+#include <linux/compat.h>
 #include <linux/cpu.h>
 #include <linux/cpu_pm.h>
 #include <linux/kernel.h>
 #include <linux/linkage.h>
+#include <linux/irqflags.h>
 #include <linux/init.h>
 #include <linux/percpu.h>
 #include <linux/preempt.h>
+#include <linux/ptrace.h>
 #include <linux/sched/signal.h>
+#include <linux/sched/task_stack.h>
 #include <linux/signal.h>
+#include <linux/slab.h>
 
 #include <asm/fpsimd.h>
 #include <asm/cputype.h>
 #include <asm/simd.h>
+#include <asm/sigcontext.h>
+#include <asm/sysreg.h>
+#include <asm/traps.h>
 
 #define FPEXC_IOF	(1 << 0)
 #define FPEXC_DZF	(1 << 1)
@@ -40,6 +49,8 @@
 #define FPEXC_IDF	(1 << 7)
 
 /*
+ * (Note: in this discussion, statements about FPSIMD apply equally to SVE.)
+ *
  * In order to reduce the number of times the FPSIMD state is needlessly saved
  * and restored, we need to keep track of two things:
  * (a) for each task, we need to remember which CPU was the last one to have
@@ -100,6 +111,279 @@
  */
 static DEFINE_PER_CPU(struct fpsimd_state *, fpsimd_last_state);
 
+/*
+ * Call __sve_free() directly only if you know task can't be scheduled
+ * or preempted.
+ */
+static void __sve_free(struct task_struct *task)
+{
+	kfree(task->thread.sve_state);
+	task->thread.sve_state = NULL;
+}
+
+static void sve_free(struct task_struct *task)
+{
+	WARN_ON(test_tsk_thread_flag(task, TIF_SVE));
+
+	__sve_free(task);
+}
+
+
+/* Offset of FFR in the SVE register dump */
+static size_t sve_ffr_offset(int vl)
+{
+	return SVE_SIG_FFR_OFFSET(sve_vq_from_vl(vl)) - SVE_SIG_REGS_OFFSET;
+}
+
+static void *sve_pffr(struct task_struct *task)
+{
+	return (char *)task->thread.sve_state +
+		sve_ffr_offset(task->thread.sve_vl);
+}
+
+static void change_cpacr(u64 val, u64 mask)
+{
+	u64 cpacr = read_sysreg(CPACR_EL1);
+	u64 new = (cpacr & ~mask) | val;
+
+	if (new != cpacr)
+		write_sysreg(new, CPACR_EL1);
+}
+
+static void sve_user_disable(void)
+{
+	change_cpacr(0, CPACR_EL1_ZEN_EL0EN);
+}
+
+static void sve_user_enable(void)
+{
+	change_cpacr(CPACR_EL1_ZEN_EL0EN, CPACR_EL1_ZEN_EL0EN);
+}
+
+/*
+ * TIF_SVE controls whether a task can use SVE without trapping while
+ * in userspace, and also the way a task's FPSIMD/SVE state is stored
+ * in thread_struct.
+ *
+ * The kernel uses this flag to track whether a user task is actively
+ * using SVE, and therefore whether full SVE register state needs to
+ * be tracked.  If not, the cheaper FPSIMD context handling code can
+ * be used instead of the more costly SVE equivalents.
+ *
+ *  * TIF_SVE set:
+ *
+ *    The task can execute SVE instructions while in userspace without
+ *    trapping to the kernel.
+ *
+ *    When stored, Z0-Z31 (incorporating Vn in bits[127:0] or the
+ *    corresponding Zn), P0-P15 and FFR are encoded in in
+ *    task->thread.sve_state, formatted appropriately for vector
+ *    length task->thread.sve_vl.
+ *
+ *    task->thread.sve_state must point to a valid buffer at least
+ *    sve_state_size(task) bytes in size.
+ *
+ *    During any syscall, the kernel may optionally clear TIF_SVE and
+ *    discard the vector state except for the FPSIMD subset.
+ *
+ *  * TIF_SVE clear:
+ *
+ *    An attempt by the user task to execute an SVE instruction causes
+ *    do_sve_acc() to be called, which does some preparation and then
+ *    sets TIF_SVE.
+ *
+ *    When stored, FPSIMD registers V0-V31 are encoded in
+ *    task->fpsimd_state; bits [max : 128] for each of Z0-Z31 are
+ *    logically zero but not stored anywhere; P0-P15 and FFR are not
+ *    stored and have unspecified values from userspace's point of
+ *    view.  For hygiene purposes, the kernel zeroes them on next use,
+ *    but userspace is discouraged from relying on this.
+ *
+ *    task->thread.sve_state does not need to be non-NULL, valid or any
+ *    particular size: it must not be dereferenced.
+ *
+ *  * FPSR and FPCR are always stored in task->fpsimd_state irrespctive of
+ *    whether TIF_SVE is clear or set, since these are not vector length
+ *    dependent.
+ */
+
+/*
+ * Update current's FPSIMD/SVE registers from thread_struct.
+ *
+ * This function should be called only when the FPSIMD/SVE state in
+ * thread_struct is known to be up to date, when preparing to enter
+ * userspace.
+ *
+ * Softirqs (and preemption) must be disabled.
+ */
+static void task_fpsimd_load(void)
+{
+	WARN_ON(!in_softirq() && !irqs_disabled());
+
+	if (system_supports_sve() && test_thread_flag(TIF_SVE))
+		sve_load_state(sve_pffr(current),
+			       &current->thread.fpsimd_state.fpsr,
+			       sve_vq_from_vl(current->thread.sve_vl) - 1);
+	else
+		fpsimd_load_state(&current->thread.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 */
+	}
+}
+
+/*
+ * Ensure current's FPSIMD/SVE storage in thread_struct is up to date
+ * with respect to the CPU registers.
+ *
+ * Softirqs (and preemption) must be disabled.
+ */
+static void task_fpsimd_save(void)
+{
+	WARN_ON(!in_softirq() && !irqs_disabled());
+
+	if (!test_thread_flag(TIF_FOREIGN_FPSTATE)) {
+		if (system_supports_sve() && test_thread_flag(TIF_SVE)) {
+			if (WARN_ON(sve_get_vl() != current->thread.sve_vl)) {
+				/*
+				 * Can't save the user regs, so current would
+				 * re-enter user with corrupt state.
+				 * There's no way to recover, so kill it:
+				 */
+				force_signal_inject(
+					SIGKILL, 0, current_pt_regs(), 0);
+				return;
+			}
+
+			sve_save_state(sve_pffr(current),
+				       &current->thread.fpsimd_state.fpsr);
+		} else
+			fpsimd_save_state(&current->thread.fpsimd_state);
+	}
+}
+
+#define ZREG(sve_state, vq, n) ((char *)(sve_state) +		\
+	(SVE_SIG_ZREG_OFFSET(vq, n) - SVE_SIG_REGS_OFFSET))
+
+/*
+ * Transfer the FPSIMD state in task->thread.fpsimd_state to
+ * task->thread.sve_state.
+ *
+ * Task can be a non-runnable task, or current.  In the latter case,
+ * softirqs (and preemption) must be disabled.
+ * task->thread.sve_state must point to at least sve_state_size(task)
+ * bytes of allocated kernel memory.
+ * task->thread.fpsimd_state must be up to date before calling this function.
+ */
+static void fpsimd_to_sve(struct task_struct *task)
+{
+	unsigned int vq;
+	void *sst = task->thread.sve_state;
+	struct fpsimd_state const *fst = &task->thread.fpsimd_state;
+	unsigned int i;
+
+	if (!system_supports_sve())
+		return;
+
+	vq = sve_vq_from_vl(task->thread.sve_vl);
+	for (i = 0; i < 32; ++i)
+		memcpy(ZREG(sst, vq, i), &fst->vregs[i],
+		       sizeof(fst->vregs[i]));
+}
+
+#ifdef CONFIG_ARM64_SVE
+
+/*
+ * Return how many bytes of memory are required to store the full SVE
+ * state for task, given task's currently configured vector length.
+ */
+size_t sve_state_size(struct task_struct const *task)
+{
+	return SVE_SIG_REGS_SIZE(sve_vq_from_vl(task->thread.sve_vl));
+}
+
+/*
+ * Ensure that task->thread.sve_state is allocated and sufficiently large.
+ *
+ * This function should be used only in preparation for replacing
+ * task->thread.sve_state with new data.  The memory is always zeroed
+ * here to prevent stale data from showing through: this is done in
+ * the interest of testability and predictability: except in the
+ * do_sve_acc() case, there is no ABI requirement to hide stale data
+ * written previously be task.
+ */
+void sve_alloc(struct task_struct *task)
+{
+	if (task->thread.sve_state) {
+		memset(task->thread.sve_state, 0, sve_state_size(current));
+		return;
+	}
+
+	/* This is a small allocation (maximum ~8KB) and Should Not Fail. */
+	task->thread.sve_state =
+		kzalloc(sve_state_size(task), GFP_KERNEL);
+
+	/*
+	 * If future SVE revisions can have larger vectors though,
+	 * this may cease to be true:
+	 */
+	BUG_ON(!task->thread.sve_state);
+}
+
+/*
+ * Called from the put_task_struct() path, which cannot get here
+ * unless dead_task is really dead and not schedulable.
+ */
+void fpsimd_release_task(struct task_struct *dead_task)
+{
+	__sve_free(dead_task);
+}
+
+#endif /* CONFIG_ARM64_SVE */
+
+/*
+ * Trapped SVE access
+ *
+ * Storage is allocated for the full SVE state, the current FPSIMD
+ * register contents are migrated across, and TIF_SVE is set so that
+ * the SVE access trap will be disabled the next time this task
+ * reaches ret_to_user.
+ *
+ * TIF_SVE should be clear on entry: otherwise, task_fpsimd_load()
+ * would have disabled the SVE access trap for userspace during
+ * ret_to_user, making an SVE access trap impossible in that case.
+ */
+asmlinkage void do_sve_acc(unsigned int esr, struct pt_regs *regs)
+{
+	/* Even if we chose not to use SVE, the hardware could still trap: */
+	if (unlikely(!system_supports_sve()) || WARN_ON(is_compat_task())) {
+		force_signal_inject(SIGILL, ILL_ILLOPC, regs, 0);
+		return;
+	}
+
+	sve_alloc(current);
+
+	local_bh_disable();
+
+	task_fpsimd_save();
+	fpsimd_to_sve(current);
+
+	/* Force ret_to_user to reload the registers: */
+	fpsimd_flush_task_state(current);
+	set_thread_flag(TIF_FOREIGN_FPSTATE);
+
+	if (test_and_set_thread_flag(TIF_SVE))
+		WARN_ON(1); /* SVE access shouldn't have trapped */
+
+	local_bh_enable();
+}
+
 /*
  * Trapped FP/ASIMD access.
  */
@@ -145,8 +429,8 @@ void fpsimd_thread_switch(struct task_struct *next)
 	 * the registers is in fact the most recent userland FPSIMD state of
 	 * 'current'.
 	 */
-	if (current->mm && !test_thread_flag(TIF_FOREIGN_FPSTATE))
-		fpsimd_save_state(&current->thread.fpsimd_state);
+	if (current->mm)
+		task_fpsimd_save();
 
 	if (next->mm) {
 		/*
@@ -168,6 +452,8 @@ void fpsimd_thread_switch(struct task_struct *next)
 
 void fpsimd_flush_thread(void)
 {
+	int vl;
+
 	if (!system_supports_fpsimd())
 		return;
 
@@ -175,6 +461,30 @@ void fpsimd_flush_thread(void)
 
 	memset(&current->thread.fpsimd_state, 0, sizeof(struct fpsimd_state));
 	fpsimd_flush_task_state(current);
+
+	if (system_supports_sve()) {
+		clear_thread_flag(TIF_SVE);
+		sve_free(current);
+
+		/*
+		 * Reset the task vector length as required.
+		 * This is where we ensure that all user tasks have a valid
+		 * vector length configured: no kernel task can become a user
+		 * task without an exec and hence a call to this function.
+		 * 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;
+
+		if (WARN_ON(!sve_vl_valid(vl)))
+			vl = SVE_VL_MIN;
+
+		current->thread.sve_vl = vl;
+	}
+
 	set_thread_flag(TIF_FOREIGN_FPSTATE);
 
 	local_bh_enable();
@@ -183,6 +493,9 @@ void fpsimd_flush_thread(void)
 /*
  * Save the userland FPSIMD state of 'current' to memory, but only if the state
  * currently held in the registers does in fact belong to 'current'
+ *
+ * Currently, SVE tasks can't exist, so just WARN in that case.
+ * Subsequent patches will add full SVE support here.
  */
 void fpsimd_preserve_current_state(void)
 {
@@ -194,6 +507,8 @@ void fpsimd_preserve_current_state(void)
 	if (!test_thread_flag(TIF_FOREIGN_FPSTATE))
 		fpsimd_save_state(&current->thread.fpsimd_state);
 
+	WARN_ON_ONCE(test_and_clear_thread_flag(TIF_SVE));
+
 	local_bh_enable();
 }
 
@@ -212,7 +527,7 @@ void fpsimd_restore_current_state(void)
 	if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
 		struct fpsimd_state *st = &current->thread.fpsimd_state;
 
-		fpsimd_load_state(st);
+		task_fpsimd_load();
 		__this_cpu_write(fpsimd_last_state, st);
 		st->cpu = smp_processor_id();
 	}
@@ -381,8 +696,8 @@ static int fpsimd_cpu_pm_notifier(struct notifier_block *self,
 {
 	switch (cmd) {
 	case CPU_PM_ENTER:
-		if (current->mm && !test_thread_flag(TIF_FOREIGN_FPSTATE))
-			fpsimd_save_state(&current->thread.fpsimd_state);
+		if (current->mm)
+			task_fpsimd_save();
 		this_cpu_write(fpsimd_last_state, NULL);
 		break;
 	case CPU_PM_EXIT:

arch/arm64/kernel/process.c

@@ -49,6 +49,7 @@
 #include <linux/notifier.h>
 #include <trace/events/power.h>
 #include <linux/percpu.h>
+#include <linux/thread_info.h>
 
 #include <asm/alternative.h>
 #include <asm/compat.h>
@@ -273,11 +274,27 @@ void release_thread(struct task_struct *dead_task)
 {
 }
 
+void arch_release_task_struct(struct task_struct *tsk)
+{
+	fpsimd_release_task(tsk);
+}
+
+/*
+ * src and dst may temporarily have aliased sve_state after task_struct
+ * is copied.  We cannot fix this properly here, because src may have
+ * live SVE state and dst's thread_info may not exist yet, so tweaking
+ * either src's or dst's TIF_SVE is not safe.
+ *
+ * The unaliasing is done in copy_thread() instead.  This works because
+ * dst is not schedulable or traceable until both of these functions
+ * have been called.
+ */
 int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 {
 	if (current->mm)
 		fpsimd_preserve_current_state();
 	*dst = *src;
+
 	return 0;
 }
 
@@ -290,6 +307,13 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
 
 	memset(&p->thread.cpu_context, 0, sizeof(struct cpu_context));
 
+	/*
+	 * Unalias p->thread.sve_state (if any) from the parent task
+	 * and disable discard SVE state for p:
+	 */
+	clear_tsk_thread_flag(p, TIF_SVE);
+	p->thread.sve_state = NULL;
+
 	if (likely(!(p->flags & PF_KTHREAD))) {
 		*childregs = *current_pt_regs();
 		childregs->regs[0] = 0;

arch/arm64/kernel/traps.c

@@ -311,7 +311,7 @@ static int call_undef_hook(struct pt_regs *regs)
 	return fn ? fn(regs, instr) : 1;
 }
 
-static void force_signal_inject(int signal, int code, struct pt_regs *regs,
+void force_signal_inject(int signal, int code, struct pt_regs *regs,
 			 unsigned long address)
 {
 	siginfo_t info;
@@ -326,7 +326,7 @@ static void force_signal_inject(int signal, int code, struct pt_regs *regs,
 		desc = "illegal memory access";
 		break;
 	default:
-		desc = "bad mode";
+		desc = "unknown or unrecoverable error";
 		break;
 	}