[PATCH v2 13/22] x86/fpu/xstate: Expand dynamic user state area on first use

[Chang S. Bae]

Intel's Extended Feature Disable (XFD) feature is an extension of the XSAVE
architecture. XFD allows the kernel to enable a feature state in XCR0 and
to receive a #NM trap when a task uses instructions accessing that state.
In this way, Linux can allocate the large task->fpu buffer only for tasks
that use it.

XFD introduces two MSRs: IA32_XFD to enable/disable the feature and
IA32_XFD_ERR to assist the #NM trap handler. Both use the same
state-component bitmap format, used by XCR0.

Use this hardware capability to find the right time to expand the xstate
area. Introduce two sets of helper functions for that:

1. The first set is primarily for interacting with the XFD hardware
   feature. Helpers for configuring disablement, e.g. in context switching,
   are:
	xdisable_setbits()
	xdisable_getbits()
	xdisable_switch()

2. The second set is for managing the first-use status and handling #NM
   trap:
	xfirstuse_enabled()
	xfirstuse_not_detected()

The #NM handler induces the xstate area expansion to save the first-used
states.

No functional change until the kernel enables dynamic user states and XFD.

Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
Change from v1:
* Inlined the XFD-induced #NM handling code (Andy Lutomirski)
---
 arch/x86/include/asm/cpufeatures.h  |  1 +
 arch/x86/include/asm/fpu/internal.h | 51 ++++++++++++++++++++++++++++-
 arch/x86/include/asm/msr-index.h    |  2 ++
 arch/x86/kernel/fpu/xstate.c        | 34 +++++++++++++++++--
 arch/x86/kernel/process.c           |  5 +++
 arch/x86/kernel/process_32.c        |  2 +-
 arch/x86/kernel/process_64.c        |  2 +-
 arch/x86/kernel/traps.c             | 40 ++++++++++++++++++++++
 8 files changed, 131 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index dad350d42ecf..5b6496ee3703 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -275,6 +275,7 @@
 #define X86_FEATURE_XSAVEC		(10*32+ 1) /* XSAVEC instruction */
 #define X86_FEATURE_XGETBV1		(10*32+ 2) /* XGETBV with ECX = 1 instruction */
 #define X86_FEATURE_XSAVES		(10*32+ 3) /* XSAVES/XRSTORS instructions */
+#define X86_FEATURE_XFD			(10*32+ 4) /* eXtended Feature Disabling */
 
 /*
  * Extended auxiliary flags: Linux defined - for features scattered in various
diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 75196d10aa71..ede3f88bdc52 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -573,11 +573,58 @@ static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
  * Misc helper functions:
  */
 
+/* The first-use detection helpers: */
+
+static inline void xdisable_setbits(u64 value)
+{
+	wrmsrl_safe(MSR_IA32_XFD, value);
+}
+
+static inline u64 xdisable_getbits(void)
+{
+	u64 value;
+
+	rdmsrl_safe(MSR_IA32_XFD, &value);
+	return value;
+}
+
+static inline u64 xfirstuse_enabled(void)
+{
+	/* All the dynamic user components are first-use enabled. */
+	return xfeatures_mask_user_dynamic;
+}
+
+/*
+ * Convert fpu->firstuse_bv to xdisable configuration in MSR IA32_XFD.
+ * xdisable_setbits() only uses this.
+ */
+static inline u64 xfirstuse_not_detected(struct fpu *fpu)
+{
+	u64 firstuse_bv = (fpu->state_mask & xfirstuse_enabled());
+
+	/*
+	 * If first-use is not detected, set the bit. If the detection is
+	 * not enabled, the bit is always zero in firstuse_bv. So, make
+	 * following conversion:
+	 */
+	return  (xfirstuse_enabled() ^ firstuse_bv);
+}
+
+/* Update MSR IA32_XFD based on fpu->firstuse_bv */
+static inline void xdisable_switch(struct fpu *prev, struct fpu *next)
+{
+	if (!static_cpu_has(X86_FEATURE_XFD) || !xfirstuse_enabled())
+		return;
+
+	if (unlikely(prev->state_mask != next->state_mask))
+		xdisable_setbits(xfirstuse_not_detected(next));
+}
+
 /*
  * Load PKRU from the FPU context if available. Delay loading of the
  * complete FPU state until the return to userland.
  */
-static inline void switch_fpu_finish(struct fpu *new_fpu)
+static inline void switch_fpu_finish(struct fpu *old_fpu, struct fpu *new_fpu)
 {
 	u32 pkru_val = init_pkru_value;
 	struct pkru_state *pk;
@@ -587,6 +634,8 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
 
 	set_thread_flag(TIF_NEED_FPU_LOAD);
 
+	xdisable_switch(old_fpu, new_fpu);
+
 	if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
 		return;
 
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 972a34d93505..f8b5f9b3c845 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -617,6 +617,8 @@
 #define MSR_IA32_BNDCFGS_RSVD		0x00000ffc
 
 #define MSR_IA32_XSS			0x00000da0
+#define MSR_IA32_XFD			0x000001c4
+#define MSR_IA32_XFD_ERR		0x000001c5
 
 #define MSR_IA32_APICBASE		0x0000001b
 #define MSR_IA32_APICBASE_BSP		(1<<8)
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index bd7c4135c1d0..bfefe8d5a7ba 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -133,6 +133,18 @@ static bool xfeature_is_supervisor(int xfeature_nr)
 	return ecx & 1;
 }
 
+static bool xfeature_disable_supported(int xfeature_nr)
+{
+	u32 eax, ebx, ecx, edx;
+
+	/*
+	 * If state component 'i' supports xfeature disable (or first-use
+	 * detection, ECX[2] return 1; otherwise, 0.
+	 */
+	cpuid_count(XSTATE_CPUID, xfeature_nr, &eax, &ebx, &ecx, &edx);
+	return ecx & 4;
+}
+
 /*
  * Available once those arrays for the offset, size, and alignment info are set up,
  * by setup_xstate_features().
@@ -316,6 +328,9 @@ void fpu__init_cpu_xstate(void)
 		wrmsrl(MSR_IA32_XSS, xfeatures_mask_supervisor() |
 				     xfeatures_mask_supervisor_dynamic());
 	}
+
+	if (boot_cpu_has(X86_FEATURE_XFD))
+		xdisable_setbits(xfirstuse_enabled());
 }
 
 static bool xfeature_enabled(enum xfeature xfeature)
@@ -515,8 +530,9 @@ static void __init print_xstate_offset_size(void)
 	for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
 		if (!xfeature_enabled(i))
 			continue;
-		pr_info("x86/fpu: xstate_offset[%d]: %4d, xstate_sizes[%d]: %4d\n",
-			 i, xstate_comp_offsets[i], i, xstate_sizes[i]);
+		pr_info("x86/fpu: xstate_offset[%d]: %4d, xstate_sizes[%d]: %4d (%s)\n",
+			i, xstate_comp_offsets[i], i, xstate_sizes[i],
+			(xfeatures_mask_user_dynamic & BIT_ULL(i)) ? "on-demand" : "default");
 	}
 }
 
@@ -947,9 +963,18 @@ void __init fpu__init_system_xstate(void)
 	}
 
 	xfeatures_mask_all &= fpu__get_supported_xfeatures_mask();
-	/* Do not support the dynamically allocated area yet. */
 	xfeatures_mask_user_dynamic = 0;
 
+	for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
+		u64 feature_mask = BIT_ULL(i);
+
+		if (!(xfeatures_mask_user() & feature_mask))
+			continue;
+
+		if (xfeature_disable_supported(i))
+			xfeatures_mask_user_dynamic |= feature_mask;
+	}
+
 	/* Enable xstate instructions to be able to continue with initialization: */
 	fpu__init_cpu_xstate();
 	err = init_xstate_size();
@@ -1004,6 +1029,9 @@ void fpu__resume_cpu(void)
 		wrmsrl(MSR_IA32_XSS, xfeatures_mask_supervisor()  |
 				     xfeatures_mask_supervisor_dynamic());
 	}
+
+	if (boot_cpu_has(X86_FEATURE_XFD))
+		xdisable_setbits(xfirstuse_not_detected(&current->thread.fpu));
 }
 
 /*
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 43d38bd09fb1..66850b63fe66 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -102,6 +102,11 @@ void arch_thread_struct_whitelist(unsigned long *offset, unsigned long *size)
 	*size = fpu_kernel_xstate_default_size;
 }
 
+void arch_release_task_struct(struct task_struct *tsk)
+{
+	free_xstate_area(&tsk->thread.fpu);
+}
+
 /*
  * Free thread data structures etc..
  */
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 4f2f54e1281c..7bd5d08eeb41 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -213,7 +213,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 
 	this_cpu_write(current_task, next_p);
 
-	switch_fpu_finish(next_fpu);
+	switch_fpu_finish(prev_fpu, next_fpu);
 
 	/* Load the Intel cache allocation PQR MSR. */
 	resctrl_sched_in();
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index df342bedea88..4f3bef245863 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -595,7 +595,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	this_cpu_write(current_task, next_p);
 	this_cpu_write(cpu_current_top_of_stack, task_top_of_stack(next_p));
 
-	switch_fpu_finish(next_fpu);
+	switch_fpu_finish(prev_fpu, next_fpu);
 
 	/* Reload sp0. */
 	update_task_stack(next_p);
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index e19df6cde35d..d2cff2622f98 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -1094,10 +1094,50 @@ DEFINE_IDTENTRY(exc_spurious_interrupt_bug)
 	 */
 }
 
+static __always_inline bool handle_xfirstuse_event(struct fpu *fpu)
+{
+	bool handled = false;
+	u64 event_mask;
+
+	/* Check whether the first-use detection is running. */
+	if (!static_cpu_has(X86_FEATURE_XFD) || !xfirstuse_enabled())
+		return handled;
+
+	rdmsrl_safe(MSR_IA32_XFD_ERR, &event_mask);
+
+	/* The trap event should happen to one of first-use enabled features */
+	WARN_ON(!(event_mask & xfirstuse_enabled()));
+
+	/* If IA32_XFD_ERR is empty, the current trap has nothing to do with. */
+	if (!event_mask)
+		return handled;
+
+	/*
+	 * The first-use event is presumed to be from userspace, so it should have
+	 * nothing to do with interrupt context.
+	 */
+	if (WARN_ON(in_interrupt()))
+		return handled;
+
+	if (alloc_xstate_area(fpu, event_mask, NULL))
+		return handled;
+
+	xdisable_setbits(xfirstuse_not_detected(fpu));
+
+	/* Clear the trap record. */
+	wrmsrl_safe(MSR_IA32_XFD_ERR, 0);
+	handled = true;
+
+	return handled;
+}
+
 DEFINE_IDTENTRY(exc_device_not_available)
 {
 	unsigned long cr0 = read_cr0();
 
+	if (handle_xfirstuse_event(&current->thread.fpu))
+		return;
+
 #ifdef CONFIG_MATH_EMULATION
 	if (!boot_cpu_has(X86_FEATURE_FPU) && (cr0 & X86_CR0_EM)) {
 		struct math_emu_info info = { };
-- 
2.17.1