arm64/fpsimd: Add task_smstop_sm()

In a few places we want to transition a task from streaming mode to
non-streaming mode, e.g. signal delivery where we historically tried to
use an SMSTOP SM instruction.

Add a new helper to manipulate a task's state in the same way as an
SMSTOP SM instruction. I have not added a corresponding helper to
simulate the effects of SMSTART SM. Only ptrace transitions a task into
streaming mode, and ptrace has distinct semantics for such transitions.

Per ARM DDI 0487 L.a, section B1.4.6:

| RRSWFQ
| When the Effective value of PSTATE.SM is changed by any method from 0
| to 1, an entry to Streaming SVE mode is performed, and all implemented
| bits of Streaming SVE register state are set to zero.

| RKFRQZ
| When the Effective value of PSTATE.SM is changed by any method from 1
| to 0, an exit from Streaming SVE mode is performed, and in the
| newly-entered mode, all implemented bits of the SVE scalable vector
| registers, SVE predicate registers, and FFR, are set to zero.

Per ARM DDI 0487 L.a, section C5.2.9:

| On entry to or exit from Streaming SVE mode, FPMR is set to 0

Per ARM DDI 0487 L.a, section C5.2.10:

| On entry to or exit from Streaming SVE mode, FPSR.{IOC, DZC, OFC, UFC,
| IXC, IDC, QC} are set to 1 and the remaining bits are set to 0.

This means bits 0, 1, 2, 3, 4, 7, and 27 respectively, i.e. 0x0800009f

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-9-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: mrutland-arm

arch/arm64/include/asm/fpsimd.h

@@ -111,6 +111,8 @@ static inline bool thread_za_enabled(struct thread_struct *thread)
 	return system_supports_sme() && (thread->svcr & SVCR_ZA_MASK);
 }
 
+extern void task_smstop_sm(struct task_struct *task);
+
 /* Maximum VL that SVE/SME VL-agnostic software can transparently support */
 #define VL_ARCH_MAX 0x100
 

arch/arm64/kernel/fpsimd.c

@@ -695,6 +695,28 @@ static inline void sve_to_fpsimd(struct task_struct *task)
 	}
 }
 
+static inline void __fpsimd_zero_vregs(struct user_fpsimd_state *fpsimd)
+{
+	memset(&fpsimd->vregs, 0, sizeof(fpsimd->vregs));
+}
+
+/*
+ * Simulate the effects of an SMSTOP SM instruction.
+ */
+void task_smstop_sm(struct task_struct *task)
+{
+	if (!thread_sm_enabled(&task->thread))
+		return;
+
+	__fpsimd_zero_vregs(&task->thread.uw.fpsimd_state);
+	task->thread.uw.fpsimd_state.fpsr = 0x0800009f;
+	if (system_supports_fpmr())
+		task->thread.uw.fpmr = 0;
+
+	task->thread.svcr &= ~SVCR_SM_MASK;
+	task->thread.fp_type = FP_STATE_FPSIMD;
+}
+
 void cpu_enable_fpmr(const struct arm64_cpu_capabilities *__always_unused p)
 {
 	write_sysreg_s(read_sysreg_s(SYS_SCTLR_EL1) | SCTLR_EL1_EnFPM_MASK,