linux-kernel - [PATCH 2/4] x86/fpu: Add a helper to prepare AMX state for CPU idle

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Date:   Thu,  4 Nov 2021 15:52:24 -0700
From:   "Chang S. Bae" <chang.seok.bae@...el.com>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, tglx@...utronix.de, dave.hansen@...ux.intel.com,
        peterz@...radead.org, bp@...en8.de, mingo@...hat.com,
        chang.seok.bae@...el.com
Subject: [PATCH 2/4] x86/fpu: Add a helper to prepare AMX state for CPU idle

When a CPU enters an idle state, non-initialized states left in large
registers may be the cause of preventing deeper low-power states.

The new helper ensures the AMX state is initialized to make the CPU
ready for low-power states.

Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Cc: x86@...nel.org
Cc: linux-kernel@...r.kernel.org
---
 arch/x86/include/asm/fpu/api.h       |  2 ++
 arch/x86/include/asm/special_insns.h |  9 +++++++++
 arch/x86/kernel/fpu/core.c           | 21 +++++++++++++++++++++
 3 files changed, 32 insertions(+)

diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
index b7267b9e452f..c4965d2d0bb6 100644
--- a/arch/x86/include/asm/fpu/api.h
+++ b/arch/x86/include/asm/fpu/api.h
@@ -162,4 +162,6 @@ static inline bool fpstate_is_confidential(struct fpu_guest *gfpu)
 struct task_struct;
 extern long fpu_xstate_prctl(struct task_struct *tsk, int option, unsigned long arg2);
 
+extern void fpu_idle_fpregs(void);
+
 #endif /* _ASM_X86_FPU_API_H */
diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 68c257a3de0d..d434fbaeb3ff 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -294,6 +294,15 @@ static inline int enqcmds(void __iomem *dst, const void *src)
 	return 0;
 }
 
+static inline void tile_release(void)
+{
+	/*
+	 * Instruction opcode for TILERELEASE; supported in binutils
+	 * version >= 2.36.
+	 */
+	asm volatile(".byte 0xc4, 0xe2, 0x78, 0x49, 0xc0");
+}
+
 #endif /* __KERNEL__ */
 
 #endif /* _ASM_X86_SPECIAL_INSNS_H */
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 8ea306b1bf8e..15d7bd67715f 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -753,3 +753,24 @@ int fpu__exception_code(struct fpu *fpu, int trap_nr)
 	 */
 	return 0;
 }
+
+/*
+ * Leaving state in some large registers may prevent the processor from
+ * entering lower-power idle states. Initialize those states when needed.
+ *
+ * A caller needs to make sure fpregs are saved before this.
+ */
+void fpu_idle_fpregs(void)
+{
+	/*
+	 * Ensure AMX TILE registers in INIT-state before entering the idle
+	 * state.
+	 *
+	 * Dynamic states are enabled only when X86_FEATURE_XGETBV1 is
+	 * available.
+	 */
+	if (fpu_state_size_dynamic() && (xfeatures_in_use() & XFEATURE_MASK_XTILE)) {
+		tile_release();
+		fpregs_deactivate(&current->thread.fpu);
+	}
+}
-- 
2.17.1