linux-kernel - [tip: x86/fpu] x86/fpu: Add a helper to prepare AMX state for low-power CPU idle

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Message-ID: <165524681334.4207.8833115136296675815.tip-bot2@tip-bot2>
Date:   Tue, 14 Jun 2022 22:46:53 -0000
From:   "tip-bot2 for Chang S. Bae" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Chang S. Bae" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/fpu] x86/fpu: Add a helper to prepare AMX state for
 low-power CPU idle

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     012b91af28e4d83240e053e16af3528a902b0d84
Gitweb:        https://git.kernel.org/tip/012b91af28e4d83240e053e16af3528a902b0d84
Author:        Chang S. Bae <chang.seok.bae@...el.com>
AuthorDate:    Wed, 08 Jun 2022 09:47:47 -07:00
Committer:     Dave Hansen <dave.hansen@...ux.intel.com>
CommitterDate: Tue, 14 Jun 2022 15:42:41 -07:00

x86/fpu: Add a helper to prepare AMX state for low-power CPU idle

When a CPU enters an idle state, a non-initialized AMX register state may
be the cause of preventing a deeper low-power state. Other extended
register states whether initialized or not do not impact the CPU idle
state.

The new helper can ensure the AMX state is initialized before the CPU is
idle, and it will be used by the intel idle driver.

Check the AMX_TILE feature bit before using XGETBV1 as a chain of
dependencies was established via cpuid_deps[]: AMX->XFD->XGETBV1.

Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Link: https://lkml.kernel.org/r/20220608164748.11864-2-chang.seok.bae@intel.com
---
 arch/x86/include/asm/fpu/api.h       |  2 ++
 arch/x86/include/asm/special_insns.h |  9 +++++++++
 arch/x86/kernel/fpu/core.c           | 14 ++++++++++++++
 3 files changed, 25 insertions(+)

diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
index 6b0f31f..503a577 100644
--- a/arch/x86/include/asm/fpu/api.h
+++ b/arch/x86/include/asm/fpu/api.h
@@ -164,4 +164,6 @@ static inline bool fpstate_is_confidential(struct fpu_guest *gfpu)
 /* prctl */
 extern long fpu_xstate_prctl(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 45b18eb..35f709f 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -295,6 +295,15 @@ 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 0fdc807..8fbbe89 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -851,3 +851,17 @@ int fpu__exception_code(struct fpu *fpu, int trap_nr)
 	 */
 	return 0;
 }
+
+/*
+ * Initialize register state that may prevent from entering low-power idle.
+ * This function will be invoked from the cpuidle driver only when needed.
+ */
+void fpu_idle_fpregs(void)
+{
+	/* Note: AMX_TILE being enabled implies XGETBV1 support */
+	if (cpu_feature_enabled(X86_FEATURE_AMX_TILE) &&
+	    (xfeatures_in_use() & XFEATURE_MASK_XTILE)) {
+		tile_release();
+		fpregs_deactivate(&current->thread.fpu);
+	}
+}