x86, fpu: decouple non-lazy/eager fpu restore from xsave

Decouple non-lazy/eager fpu restore policy from the existence of the xsave
feature. Introduce a synthetic CPUID flag to represent the eagerfpu
policy. "eagerfpu=on" boot paramter will enable the policy.

Requested-by: H. Peter Anvin <hpa@zytor.com>
Requested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Link: http://lkml.kernel.org/r/1347300665-6209-2-git-send-email-suresh.b.siddha@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

Documentation/kernel-parameters.txt

@@ -1833,6 +1833,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			and restore using xsave. The kernel will fallback to
 			enabling legacy floating-point and sse state.
 
+	eagerfpu=	[X86]
+			on	enable eager fpu restore
+			off	disable eager fpu restore
+
 	nohlt		[BUGS=ARM,SH] Tells the kernel that the sleep(SH) or
 			wfi(ARM) instruction doesn't work correctly and not to
 			use it. This is also useful when using JTAG debugger.

arch/x86/include/asm/cpufeature.h

@@ -97,6 +97,7 @@
 #define X86_FEATURE_EXTD_APICID	(3*32+26) /* has extended APICID (8 bits) */
 #define X86_FEATURE_AMD_DCM     (3*32+27) /* multi-node processor */
 #define X86_FEATURE_APERFMPERF	(3*32+28) /* APERFMPERF */
+#define X86_FEATURE_EAGER_FPU	(3*32+29) /* "eagerfpu" Non lazy FPU restore */
 
 /* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
 #define X86_FEATURE_XMM3	(4*32+ 0) /* "pni" SSE-3 */
@@ -305,6 +306,7 @@ extern const char * const x86_power_flags[32];
 #define cpu_has_perfctr_core	boot_cpu_has(X86_FEATURE_PERFCTR_CORE)
 #define cpu_has_cx8		boot_cpu_has(X86_FEATURE_CX8)
 #define cpu_has_cx16		boot_cpu_has(X86_FEATURE_CX16)
+#define cpu_has_eager_fpu	boot_cpu_has(X86_FEATURE_EAGER_FPU)
 
 #if defined(CONFIG_X86_INVLPG) || defined(CONFIG_X86_64)
 # define cpu_has_invlpg		1

arch/x86/include/asm/fpu-internal.h

@@ -38,6 +38,7 @@ int ia32_setup_frame(int sig, struct k_sigaction *ka,
 
 extern unsigned int mxcsr_feature_mask;
 extern void fpu_init(void);
+extern void eager_fpu_init(void);
 
 DECLARE_PER_CPU(struct task_struct *, fpu_owner_task);
 
@@ -84,6 +85,11 @@ static inline int is_x32_frame(void)
 
 #define X87_FSW_ES (1 << 7)	/* Exception Summary */
 
+static __always_inline __pure bool use_eager_fpu(void)
+{
+	return static_cpu_has(X86_FEATURE_EAGER_FPU);
+}
+
 static __always_inline __pure bool use_xsaveopt(void)
 {
 	return static_cpu_has(X86_FEATURE_XSAVEOPT);
@@ -99,6 +105,14 @@ static __always_inline __pure bool use_fxsr(void)
         return static_cpu_has(X86_FEATURE_FXSR);
 }
 
+static inline void fx_finit(struct i387_fxsave_struct *fx)
+{
+	memset(fx, 0, xstate_size);
+	fx->cwd = 0x37f;
+	if (cpu_has_xmm)
+		fx->mxcsr = MXCSR_DEFAULT;
+}
+
 extern void __sanitize_i387_state(struct task_struct *);
 
 static inline void sanitize_i387_state(struct task_struct *tsk)
@@ -291,13 +305,13 @@ static inline void __thread_set_has_fpu(struct task_struct *tsk)
 static inline void __thread_fpu_end(struct task_struct *tsk)
 {
 	__thread_clear_has_fpu(tsk);
-	if (!use_xsave())
+	if (!use_eager_fpu())
 		stts();
 }
 
 static inline void __thread_fpu_begin(struct task_struct *tsk)
 {
-	if (!use_xsave())
+	if (!use_eager_fpu())
 		clts();
 	__thread_set_has_fpu(tsk);
 }
@@ -327,10 +341,14 @@ static inline void drop_fpu(struct task_struct *tsk)
 
 static inline void drop_init_fpu(struct task_struct *tsk)
 {
-	if (!use_xsave())
+	if (!use_eager_fpu())
 		drop_fpu(tsk);
-	else
+	else {
+		if (use_xsave())
 			xrstor_state(init_xstate_buf, -1);
+		else
+			fxrstor_checking(&init_xstate_buf->i387);
+	}
 }
 
 /*
@@ -370,7 +388,7 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
 	 * If the task has used the math, pre-load the FPU on xsave processors
 	 * or if the past 5 consecutive context-switches used math.
 	 */
-	fpu.preload = tsk_used_math(new) && (use_xsave() ||
+	fpu.preload = tsk_used_math(new) && (use_eager_fpu() ||
 					     new->fpu_counter > 5);
 	if (__thread_has_fpu(old)) {
 		if (!__save_init_fpu(old))
@@ -383,14 +401,14 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
 			new->fpu_counter++;
 			__thread_set_has_fpu(new);
 			prefetch(new->thread.fpu.state);
-		} else if (!use_xsave())
+		} else if (!use_eager_fpu())
 			stts();
 	} else {
 		old->fpu_counter = 0;
 		old->thread.fpu.last_cpu = ~0;
 		if (fpu.preload) {
 			new->fpu_counter++;
-			if (!use_xsave() && fpu_lazy_restore(new, cpu))
+			if (!use_eager_fpu() && fpu_lazy_restore(new, cpu))
 				fpu.preload = 0;
 			else
 				prefetch(new->thread.fpu.state);
@@ -452,6 +470,14 @@ static inline void user_fpu_begin(void)
 	preempt_enable();
 }
 
+static inline void __save_fpu(struct task_struct *tsk)
+{
+	if (use_xsave())
+		xsave_state(&tsk->thread.fpu.state->xsave, -1);
+	else
+		fpu_fxsave(&tsk->thread.fpu);
+}
+
 /*
  * These disable preemption on their own and are safe
  */
@@ -459,8 +485,8 @@ static inline void save_init_fpu(struct task_struct *tsk)
 {
 	WARN_ON_ONCE(!__thread_has_fpu(tsk));
 
-	if (use_xsave()) {
+	if (use_eager_fpu()) {
-		xsave_state(&tsk->thread.fpu.state->xsave, -1);
+		__save_fpu(tsk);
 		return;
 	}
 
@@ -526,11 +552,9 @@ static inline void fpu_free(struct fpu *fpu)
 
 static inline void fpu_copy(struct task_struct *dst, struct task_struct *src)
 {
-	if (use_xsave()) {
+	if (use_eager_fpu()) {
-		struct xsave_struct *xsave = &dst->thread.fpu.state->xsave;
+		memset(&dst->thread.fpu.state->xsave, 0, xstate_size);
-
+		__save_fpu(dst);
-		memset(&xsave->xsave_hdr, 0, sizeof(struct xsave_hdr_struct));
-		xsave_state(xsave, -1);
 	} else {
 		struct fpu *dfpu = &dst->thread.fpu;
 		struct fpu *sfpu = &src->thread.fpu;

arch/x86/kernel/cpu/common.c

@@ -1297,7 +1297,6 @@ void __cpuinit cpu_init(void)
 	dbg_restore_debug_regs();
 
 	fpu_init();
-	xsave_init();
 
 	raw_local_save_flags(kernel_eflags);
 
@@ -1352,6 +1351,5 @@ void __cpuinit cpu_init(void)
 	dbg_restore_debug_regs();
 
 	fpu_init();
-	xsave_init();
 }
 #endif

arch/x86/kernel/i387.c

@@ -22,9 +22,8 @@
 /*
  * Were we in an interrupt that interrupted kernel mode?
  *
- * For now, on xsave platforms we will return interrupted
+ * For now, with eagerfpu we will return interrupted kernel FPU
- * kernel FPU as not-idle. TBD: As we use non-lazy FPU restore
+ * state as not-idle. TBD: Ideally we can change the return value
- * for xsave platforms, ideally we can change the return value
  * to something like __thread_has_fpu(current). But we need to
  * be careful of doing __thread_clear_has_fpu() before saving
  * the FPU etc for supporting nested uses etc. For now, take
@@ -38,7 +37,7 @@
  */
 static inline bool interrupted_kernel_fpu_idle(void)
 {
-	if (use_xsave())
+	if (use_eager_fpu())
 		return 0;
 
 	return !__thread_has_fpu(current) &&
@@ -84,7 +83,7 @@ void kernel_fpu_begin(void)
 		__save_init_fpu(me);
 		__thread_clear_has_fpu(me);
 		/* We do 'stts()' in kernel_fpu_end() */
-	} else if (!use_xsave()) {
+	} else if (!use_eager_fpu()) {
 		this_cpu_write(fpu_owner_task, NULL);
 		clts();
 	}
@@ -93,7 +92,7 @@ EXPORT_SYMBOL(kernel_fpu_begin);
 
 void kernel_fpu_end(void)
 {
-	if (use_xsave())
+	if (use_eager_fpu())
 		math_state_restore();
 	else
 		stts();
@@ -122,7 +121,6 @@ static void __cpuinit mxcsr_feature_mask_init(void)
 {
 	unsigned long mask = 0;
 
-	clts();
 	if (cpu_has_fxsr) {
 		memset(&fx_scratch, 0, sizeof(struct i387_fxsave_struct));
 		asm volatile("fxsave %0" : : "m" (fx_scratch));
@@ -131,7 +129,6 @@ static void __cpuinit mxcsr_feature_mask_init(void)
 			mask = 0x0000ffbf;
 	}
 	mxcsr_feature_mask &= mask;
-	stts();
 }
 
 static void __cpuinit init_thread_xstate(void)
@@ -185,9 +182,8 @@ void __cpuinit fpu_init(void)
 		init_thread_xstate();
 
 	mxcsr_feature_mask_init();
-	/* clean state in init */
+	xsave_init();
-	current_thread_info()->status = 0;
+	eager_fpu_init();
-	clear_used_math();
 }
 
 void fpu_finit(struct fpu *fpu)
@@ -198,12 +194,7 @@ void fpu_finit(struct fpu *fpu)
 	}
 
 	if (cpu_has_fxsr) {
-		struct i387_fxsave_struct *fx = &fpu->state->fxsave;
+		fx_finit(&fpu->state->fxsave);
-
-		memset(fx, 0, xstate_size);
-		fx->cwd = 0x37f;
-		if (cpu_has_xmm)
-			fx->mxcsr = MXCSR_DEFAULT;
 	} else {
 		struct i387_fsave_struct *fp = &fpu->state->fsave;
 		memset(fp, 0, xstate_size);

arch/x86/kernel/process.c

@@ -156,7 +156,7 @@ void flush_thread(void)
 	 * Free the FPU state for non xsave platforms. They get reallocated
 	 * lazily at the first use.
 	 */
-	if (!use_xsave())
+	if (!use_eager_fpu())
 		free_thread_xstate(tsk);
 }
 

arch/x86/kernel/traps.c

@@ -630,7 +630,7 @@ EXPORT_SYMBOL_GPL(math_state_restore);
 dotraplinkage void __kprobes
 do_device_not_available(struct pt_regs *regs, long error_code)
 {
-	BUG_ON(use_xsave());
+	BUG_ON(use_eager_fpu());
 
 #ifdef CONFIG_MATH_EMULATION
 	if (read_cr0() & X86_CR0_EM) {

arch/x86/kernel/xsave.c

@@ -400,7 +400,7 @@ int __restore_xstate_sig(void __user *buf, void __user *buf_fx, int size)
 			set_used_math();
 		}
 
-		if (use_xsave())
+		if (use_eager_fpu())
 			math_state_restore();
 
 		return err;
@@ -450,28 +450,10 @@ static void prepare_fx_sw_frame(void)
  */
 static inline void xstate_enable(void)
 {
-	clts();
 	set_in_cr4(X86_CR4_OSXSAVE);
 	xsetbv(XCR_XFEATURE_ENABLED_MASK, pcntxt_mask);
 }
 
-/*
- * This is same as math_state_restore(). But use_xsave() is not yet
- * patched to use math_state_restore().
- */
-static inline void init_restore_xstate(void)
-{
-	init_fpu(current);
-	__thread_fpu_begin(current);
-	xrstor_state(init_xstate_buf, -1);
-}
-
-static inline void xstate_enable_ap(void)
-{
-	xstate_enable();
-	init_restore_xstate();
-}
-
 /*
  * Record the offsets and sizes of different state managed by the xsave
  * memory layout.
@@ -500,17 +482,20 @@ static void __init setup_xstate_features(void)
 /*
  * setup the xstate image representing the init state
  */
-static void __init setup_xstate_init(void)
+static void __init setup_init_fpu_buf(void)
 {
-	setup_xstate_features();
-
 	/*
 	 * Setup init_xstate_buf to represent the init state of
 	 * all the features managed by the xsave
 	 */
 	init_xstate_buf = alloc_bootmem_align(xstate_size,
 					      __alignof__(struct xsave_struct));
-	init_xstate_buf->i387.mxcsr = MXCSR_DEFAULT;
+	fx_finit(&init_xstate_buf->i387);
+
+	if (!cpu_has_xsave)
+		return;
+
+	setup_xstate_features();
 
 	/*
 	 * Init all the features state with header_bv being 0x0
@@ -523,6 +508,17 @@ static void __init setup_xstate_init(void)
 	xsave_state(init_xstate_buf, -1);
 }
 
+static int disable_eagerfpu;
+static int __init eager_fpu_setup(char *s)
+{
+	if (!strcmp(s, "on"))
+		setup_force_cpu_cap(X86_FEATURE_EAGER_FPU);
+	else if (!strcmp(s, "off"))
+		disable_eagerfpu = 1;
+	return 1;
+}
+__setup("eagerfpu=", eager_fpu_setup);
+
 /*
  * Enable and initialize the xsave feature.
  */
@@ -559,15 +555,10 @@ static void __init xstate_enable_boot_cpu(void)
 
 	update_regset_xstate_info(xstate_size, pcntxt_mask);
 	prepare_fx_sw_frame();
-
+	setup_init_fpu_buf();
-	setup_xstate_init();
 
 	pr_info("enabled xstate_bv 0x%llx, cntxt size 0x%x\n",
 		pcntxt_mask, xstate_size);
-
-	current->thread.fpu.state =
-	     alloc_bootmem_align(xstate_size, __alignof__(struct xsave_struct));
-	init_restore_xstate();
 }
 
 /*
@@ -586,6 +577,42 @@ void __cpuinit xsave_init(void)
 		return;
 
 	this_func = next_func;
-	next_func = xstate_enable_ap;
+	next_func = xstate_enable;
 	this_func();
 }
+
+static inline void __init eager_fpu_init_bp(void)
+{
+	current->thread.fpu.state =
+	    alloc_bootmem_align(xstate_size, __alignof__(struct xsave_struct));
+	if (!init_xstate_buf)
+		setup_init_fpu_buf();
+}
+
+void __cpuinit eager_fpu_init(void)
+{
+	static __refdata void (*boot_func)(void) = eager_fpu_init_bp;
+
+	clear_used_math();
+	current_thread_info()->status = 0;
+	if (!cpu_has_eager_fpu) {
+		stts();
+		return;
+	}
+
+	if (boot_func) {
+		boot_func();
+		boot_func = NULL;
+	}
+
+	/*
+	 * This is same as math_state_restore(). But use_xsave() is
+	 * not yet patched to use math_state_restore().
+	 */
+	init_fpu(current);
+	__thread_fpu_begin(current);
+	if (cpu_has_xsave)
+		xrstor_state(init_xstate_buf, -1);
+	else
+		fxrstor_checking(&init_xstate_buf->i387);
+}