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@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20220608164748.11864-2-chang.seok.bae@intel.com

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 */

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 */

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);
+	}
+}