[v5,27/38] arm64/sme: Disable streaming mode and ZA when flushing CPU state

Commit Message

Mark Brown Oct. 27, 2021, 6:44 p.m. UTC

Both streaming mode and ZA may increase power consumption when they are
enabled and streaming mode makes many FPSIMD and SVE instructions undefined
which will cause problems for any kernel mode floating point so disable
both when we flush the CPU state. This covers both kernel_neon_begin() and
idle and after flushing the state a reload is always required anyway.

Signed-off-by: Mark Brown <[e-mail]>
---
 arch/arm64/include/asm/fpsimd.h | 7 +++++++
 arch/arm64/kernel/fpsimd.c      | 9 +++++++++
 2 files changed, 16 insertions(+)

Patch

diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
index 99b5b822c47f..a7291917218c 100644
--- a/arch/arm64/include/asm/fpsimd.h
+++ b/arch/arm64/include/asm/fpsimd.h
@@ -306,6 +306,12 @@  static inline void sme_smstop_sm(void)
 	asm volatile(".inst 0xd503427f");
 }
 
+static inline void sme_smstop(void)
+{
+	/* SMSTOP SM is an alias for MSR SVCRSMZA, #0 */
+	asm volatile(".inst 0x7f4603d5");
+}
+
 static inline int sme_max_vl(void)
 {
 	return vec_max_vl(ARM64_VEC_SME);
@@ -328,6 +334,7 @@  static inline void sme_alloc(struct task_struct *task) { }
 
 static inline void sme_smstart_sm(void) { }
 static inline void sme_smstop_sm(void) { }
+static inline void sme_smstop(void) { }
 
 static inline int sme_set_current_vl(unsigned long arg)
 {
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 69f76df08ee8..ebaf174bdf82 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -1755,6 +1755,15 @@  static void fpsimd_flush_cpu_state(void)
 {
 	WARN_ON(!system_supports_fpsimd());
 	__this_cpu_write(fpsimd_last_state.st, NULL);
+
+	/*
+	 * Leaving streaming mode enabled will cause issues for any kernel
+	 * NEON and leaving streaming mode or ZA enabled may increase power
+	 * consumption.
+	 */
+	if (system_supports_sme())
+		sme_smstop();
+
 	set_thread_flag(TIF_FOREIGN_FPSTATE);
 }