arm64/sme: Implement ZT0 ptrace support

Implement support for a new note type NT_ARM64_ZT providing access to
ZT0 when implemented. Since ZT0 is a register with constant size this is
much simpler than for other SME state.

As ZT0 is only accessible when PSTATE.ZA is set writes to ZT0 cause
PSTATE.ZA to be set, the main alternative would be to return -EBUSY in
this case but this seemed more constructive. Practical users are also
going to be working with ZA anyway and have some understanding of the
state.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-sme2-v4-12-f2fa0aef982f@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/ptrace.c

@@ -1138,6 +1138,51 @@ static int za_set(struct task_struct *target,
 	return ret;
 }
 
+static int zt_get(struct task_struct *target,
+		  const struct user_regset *regset,
+		  struct membuf to)
+{
+	if (!system_supports_sme2())
+		return -EINVAL;
+
+	/*
+	 * If PSTATE.ZA is not set then ZT will be zeroed when it is
+	 * enabled so report the current register value as zero.
+	 */
+	if (thread_za_enabled(&target->thread))
+		membuf_write(&to, thread_zt_state(&target->thread),
+			     ZT_SIG_REG_BYTES);
+	else
+		membuf_zero(&to, ZT_SIG_REG_BYTES);
+
+	return 0;
+}
+
+static int zt_set(struct task_struct *target,
+		  const struct user_regset *regset,
+		  unsigned int pos, unsigned int count,
+		  const void *kbuf, const void __user *ubuf)
+{
+	int ret;
+
+	if (!system_supports_sme2())
+		return -EINVAL;
+
+	if (!thread_za_enabled(&target->thread)) {
+		sme_alloc(target);
+		if (!target->thread.sme_state)
+			return -ENOMEM;
+	}
+
+	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
+				 thread_zt_state(&target->thread),
+				 0, ZT_SIG_REG_BYTES);
+	if (ret == 0)
+		target->thread.svcr |= SVCR_ZA_MASK;
+
+	return ret;
+}
+
 #endif /* CONFIG_ARM64_SME */
 
 #ifdef CONFIG_ARM64_PTR_AUTH
@@ -1360,6 +1405,7 @@ enum aarch64_regset {
 #ifdef CONFIG_ARM64_SVE
 	REGSET_SSVE,
 	REGSET_ZA,
+	REGSET_ZT,
 #endif
 #ifdef CONFIG_ARM64_PTR_AUTH
 	REGSET_PAC_MASK,
@@ -1467,6 +1513,14 @@ static const struct user_regset aarch64_regsets[] = {
 		.regset_get = za_get,
 		.set = za_set,
 	},
+	[REGSET_ZT] = { /* SME ZT */
+		.core_note_type = NT_ARM_ZT,
+		.n = 1,
+		.size = ZT_SIG_REG_BYTES,
+		.align = sizeof(u64),
+		.regset_get = zt_get,
+		.set = zt_set,
+	},
 #endif
 #ifdef CONFIG_ARM64_PTR_AUTH
 	[REGSET_PAC_MASK] = {

include/uapi/linux/elf.h

@@ -434,6 +434,7 @@ typedef struct elf64_shdr {
 #define NT_ARM_PAC_ENABLED_KEYS	0x40a	/* arm64 ptr auth enabled keys (prctl()) */
 #define NT_ARM_SSVE	0x40b		/* ARM Streaming SVE registers */
 #define NT_ARM_ZA	0x40c		/* ARM SME ZA registers */
+#define NT_ARM_ZT	0x40d		/* ARM SME ZT registers */
 #define NT_ARC_V2	0x600		/* ARCv2 accumulator/extra registers */
 #define NT_VMCOREDD	0x700		/* Vmcore Device Dump Note */
 #define NT_MIPS_DSP	0x800		/* MIPS DSP ASE registers */