[PATCH 22/22] x86/fpu: Defer FPU state load until return to userspace

Sebastian Andrzej Siewior
2019-01-09T11:49:03Z

From: Rik van Riel <riel@surriel.com>

Defer loading of FPU state until return to userspace. This gives
the kernel the potential to skip loading FPU state for tasks that
stay in kernel mode, or for tasks that end up with repeated
invocations of kernel_fpu_begin() & kernel_fpu_end().

The __fpregs_changes_{begin|end}() section ensures that the register
remain unchanged. Otherwise a context switch or a BH could save the
registers to its FPU context and processor's FPU register would became
random if beeing modified at the same time.

KVM swaps the host/guest register on entry/exit path. I kept the flow as
is. First it ensures that the registers are loaded and then saves the
current (host) state before it loads the guest's register. The swap is
done at the very end with disabled interrupts so it should not change
anymore before theg guest is entered. The read/save version seems to be
cheaper compared to memcpy() in a micro benchmark.

Each thread gets TIF_NEED_FPU_LOAD set as part of fork() / fpu__copy().
For kernel threads, this flag gets never cleared which avoids saving /
restoring the FPU state for kernel threads and during in-kernel usage of
the FPU register.

Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
arch/x86/entry/common.c | 8 +++
arch/x86/include/asm/fpu/api.h | 22 +++++-
arch/x86/include/asm/fpu/internal.h | 27 +++++---
arch/x86/include/asm/trace/fpu.h | 5 +-
arch/x86/kernel/fpu/core.c | 104 +++++++++++++++++++++-------
arch/x86/kernel/fpu/signal.c | 46 +++++++-----
arch/x86/kernel/process.c | 2 +-
arch/x86/kernel/process_32.c | 5 +-
arch/x86/kernel/process_64.c | 5 +-
arch/x86/kvm/x86.c | 20 ++++--
10 files changed, 179 insertions(+), 65 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 7bc105f47d21a..13e8e29af6ab7 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -31,6 +31,7 @@
#include <asm/vdso.h>
#include <linux/uaccess.h>
#include <asm/cpufeature.h>
+#include <asm/fpu/api.h>

#define CREATE_TRACE_POINTS
#include <trace/events/syscalls.h>
@@ -196,6 +197,13 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
if (unlikely(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
exit_to_usermode_loop(regs, cached_flags);

+ /* Reload ti->flags; we may have rescheduled above. */
+ cached_flags = READ_ONCE(ti->flags);
+
+ fpregs_assert_state_consistent();
+ if (unlikely(cached_flags & _TIF_NEED_FPU_LOAD))
+ switch_fpu_return();
+
#ifdef CONFIG_COMPAT
/*
* Compat syscalls set TS_COMPAT. Make sure we clear it before
diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
index 31b66af8eb914..c17620af5d797 100644
--- a/arch/x86/include/asm/fpu/api.h
+++ b/arch/x86/include/asm/fpu/api.h
@@ -10,7 +10,7 @@

#ifndef _ASM_X86_FPU_API_H
#define _ASM_X86_FPU_API_H
-#include <linux/preempt.h>
+#include <linux/bottom_half.h>

/*
* Use kernel_fpu_begin/end() if you intend to use FPU in kernel context. It
@@ -22,17 +22,37 @@
extern void kernel_fpu_begin(void);
extern void kernel_fpu_end(void);
extern bool irq_fpu_usable(void);
+extern void fpregs_mark_activate(void);

+/*
+ * Use __fpregs_changes_begin() while editing CPU's FPU registers or fpu->state.
+ * A context switch will (and softirq might) save CPU's FPU register to
+ * fpu->state and set TIF_NEED_FPU_LOAD leaving CPU's FPU registers in a random
+ * state.
+ */
static inline void __fpregs_changes_begin(void)
{
preempt_disable();
+ local_bh_disable();
}

static inline void __fpregs_changes_end(void)
{
+ local_bh_enable();
preempt_enable();
}

+#ifdef CONFIG_X86_DEBUG_FPU
+extern void fpregs_assert_state_consistent(void);
+#else
+static inline void fpregs_assert_state_consistent(void) { }
+#endif
+
+/*
+ * Load the task FPU state before returning to userspace.
+ */
+extern void switch_fpu_return(void);
+
/*
* Query the presence of one or more xfeatures. Works on any legacy CPU as well.
*
diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 672e51bc0e9b5..61627f8cb3ff4 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -29,7 +29,7 @@ extern void fpu__prepare_write(struct fpu *fpu);
extern void fpu__save(struct fpu *fpu);
extern int fpu__restore_sig(void __user *buf, int ia32_frame);
extern void fpu__drop(struct fpu *fpu);
-extern int fpu__copy(struct fpu *dst_fpu, struct fpu *src_fpu);
+extern int fpu__copy(struct task_struct *dst, struct task_struct *src);
extern void fpu__clear(struct fpu *fpu);
extern int fpu__exception_code(struct fpu *fpu, int trap_nr);
extern int dump_fpu(struct pt_regs *ptregs, struct user_i387_struct *fpstate);
@@ -482,13 +482,20 @@ static inline void fpregs_activate(struct fpu *fpu)
trace_x86_fpu_regs_activated(fpu);
}

-static inline void __fpregs_load_activate(struct fpu *fpu, int cpu)
+static inline void __fpregs_load_activate(void)
{
+ struct fpu *fpu = &current->thread.fpu;
+ int cpu = smp_processor_id();
+
+ if (WARN_ON_ONCE(current->mm == NULL))
+ return;
+
if (!fpregs_state_valid(fpu, cpu)) {
- if (current->mm)
- copy_kernel_to_fpregs(&fpu->state);
+ copy_kernel_to_fpregs(&fpu->state);
fpregs_activate(fpu);
+ fpu->last_cpu = cpu;
}
+ clear_thread_flag(TIF_NEED_FPU_LOAD);
}

/*
@@ -499,8 +506,8 @@ static inline void __fpregs_load_activate(struct fpu *fpu, int cpu)
* - switch_fpu_prepare() saves the old state.
* This is done within the context of the old process.
*
- * - switch_fpu_finish() restores the new state as
- * necessary.
+ * - switch_fpu_finish() sets TIF_NEED_FPU_LOAD; the floating point state
+ * will get loaded on return to userspace, or when the kernel needs it.
*/
static inline void
switch_fpu_prepare(struct fpu *old_fpu, int cpu)
@@ -521,10 +528,10 @@ switch_fpu_prepare(struct fpu *old_fpu, int cpu)
*/

/*
- * Set up the userspace FPU context for the new task, if the task
- * has used the FPU.
+ * Load PKRU from the FPU context if available. Delay loading the loading of the
+ * complete FPU state until the return to userland.
*/
-static inline void switch_fpu_finish(struct fpu *new_fpu, int cpu)
+static inline void switch_fpu_finish(struct fpu *new_fpu)
{
struct pkru_state *pk;
u32 pkru_val = 0;
@@ -532,7 +539,7 @@ static inline void switch_fpu_finish(struct fpu *new_fpu, int cpu)
if (!static_cpu_has(X86_FEATURE_FPU))
return;

- __fpregs_load_activate(new_fpu, cpu);
+ set_thread_flag(TIF_NEED_FPU_LOAD);

if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
return;
diff --git a/arch/x86/include/asm/trace/fpu.h b/arch/x86/include/asm/trace/fpu.h
index bd65f6ba950f8..91a1422091ceb 100644
--- a/arch/x86/include/asm/trace/fpu.h
+++ b/arch/x86/include/asm/trace/fpu.h
@@ -13,19 +13,22 @@ DECLARE_EVENT_CLASS(x86_fpu,

TP_STRUCT__entry(
__field(struct fpu *, fpu)
+ __field(bool, load_fpu)
__field(u64, xfeatures)
__field(u64, xcomp_bv)
),

TP_fast_assign(
__entry->fpu = fpu;
+ __entry->load_fpu = test_thread_flag(TIF_NEED_FPU_LOAD);
if (boot_cpu_has(X86_FEATURE_OSXSAVE)) {
__entry->xfeatures = fpu->state.xsave.header.xfeatures;
__entry->xcomp_bv = fpu->state.xsave.header.xcomp_bv;
}
),
- TP_printk("x86/fpu: %p xfeatures: %llx xcomp_bv: %llx",
+ TP_printk("x86/fpu: %p load: %d xfeatures: %llx xcomp_bv: %llx",
__entry->fpu,
+ __entry->load_fpu,
__entry->xfeatures,
__entry->xcomp_bv
)
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 78d8037635932..f52e687dff9ee 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -102,23 +102,20 @@ static void __kernel_fpu_begin(void)
kernel_fpu_disable();

if (current->mm) {
- /*
- * Ignore return value -- we don't care if reg state
- * is clobbered.
- */
- copy_fpregs_to_fpstate(fpu);
- } else {
- __cpu_invalidate_fpregs_state();
+ if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
+ set_thread_flag(TIF_NEED_FPU_LOAD);
+ /*
+ * Ignore return value -- we don't care if reg state
+ * is clobbered.
+ */
+ copy_fpregs_to_fpstate(fpu);
+ }
}
+ __cpu_invalidate_fpregs_state();
}

static void __kernel_fpu_end(void)
{
- struct fpu *fpu = &current->thread.fpu;
-
- if (current->mm)
- copy_kernel_to_fpregs(&fpu->state);
-
kernel_fpu_enable();
}

@@ -145,14 +142,16 @@ void fpu__save(struct fpu *fpu)
{
WARN_ON_FPU(fpu != &current->thread.fpu);

- preempt_disable();
+ __fpregs_changes_begin();
trace_x86_fpu_before_save(fpu);

- if (!copy_fpregs_to_fpstate(fpu)) {
- copy_kernel_to_fpregs(&fpu->state);
+ if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
+ if (!copy_fpregs_to_fpstate(fpu)) {
+ copy_kernel_to_fpregs(&fpu->state);
+ }
}
trace_x86_fpu_after_save(fpu);
- preempt_enable();
+ __fpregs_changes_end();
}
EXPORT_SYMBOL_GPL(fpu__save);

@@ -185,8 +184,11 @@ void fpstate_init(union fpregs_state *state)
}
EXPORT_SYMBOL_GPL(fpstate_init);

-int fpu__copy(struct fpu *dst_fpu, struct fpu *src_fpu)
+int fpu__copy(struct task_struct *dst, struct task_struct *src)
{
+ struct fpu *dst_fpu = &dst->thread.fpu;
+ struct fpu *src_fpu = &src->thread.fpu;
+
dst_fpu->last_cpu = -1;

if (!static_cpu_has(X86_FEATURE_FPU))
@@ -201,16 +203,23 @@ int fpu__copy(struct fpu *dst_fpu, struct fpu *src_fpu)
memset(&dst_fpu->state.xsave, 0, fpu_kernel_xstate_size);

/*
- * Save current FPU registers directly into the child
- * FPU context, without any memory-to-memory copying.
+ * If the FPU registers are not current just memcpy() the state.
+ * Otherwise save current FPU registers directly into the child's FPU
+ * context, without any memory-to-memory copying.
*
* ( The function 'fails' in the FNSAVE case, which destroys
- * register contents so we have to copy them back. )
+ * register contents so we have to load them back. )
*/
- if (!copy_fpregs_to_fpstate(dst_fpu)) {
- memcpy(&src_fpu->state, &dst_fpu->state, fpu_kernel_xstate_size);
- copy_kernel_to_fpregs(&src_fpu->state);
- }
+ __fpregs_changes_begin();
+ if (test_thread_flag(TIF_NEED_FPU_LOAD))
+ memcpy(&dst_fpu->state, &src_fpu->state, fpu_kernel_xstate_size);
+
+ else if (!copy_fpregs_to_fpstate(dst_fpu))
+ copy_kernel_to_fpregs(&dst_fpu->state);
+
+ __fpregs_changes_end();
+
+ set_tsk_thread_flag(dst, TIF_NEED_FPU_LOAD);

trace_x86_fpu_copy_src(src_fpu);
trace_x86_fpu_copy_dst(dst_fpu);
@@ -226,10 +235,9 @@ static void fpu__initialize(struct fpu *fpu)
{
WARN_ON_FPU(fpu != &current->thread.fpu);

+ set_thread_flag(TIF_NEED_FPU_LOAD);
fpstate_init(&fpu->state);
trace_x86_fpu_init_state(fpu);
-
- trace_x86_fpu_activate_state(fpu);
}

/*
@@ -308,6 +316,8 @@ void fpu__drop(struct fpu *fpu)
*/
static inline void copy_init_fpstate_to_fpregs(void)
{
+ __fpregs_changes_begin();
+
if (use_xsave())
copy_kernel_to_xregs(&init_fpstate.xsave, -1);
else if (static_cpu_has(X86_FEATURE_FXSR))
@@ -317,6 +327,9 @@ static inline void copy_init_fpstate_to_fpregs(void)

if (boot_cpu_has(X86_FEATURE_OSPKE))
copy_init_pkru_to_fpregs();
+
+ fpregs_mark_activate();
+ __fpregs_changes_end();
}

/*
@@ -339,6 +352,45 @@ void fpu__clear(struct fpu *fpu)
copy_init_fpstate_to_fpregs();
}

+/*
+ * Load FPU context before returning to userspace.
+ */
+void switch_fpu_return(void)
+{
+ if (!static_cpu_has(X86_FEATURE_FPU))
+ return;
+
+ __fpregs_load_activate();
+}
+EXPORT_SYMBOL_GPL(switch_fpu_return);
+
+#ifdef CONFIG_X86_DEBUG_FPU
+/*
+ * If current FPU state according to its tracking (loaded FPU ctx on this CPU)
+ * is not valid then we must have TIF_NEED_FPU_LOAD set so the context is loaded on
+ * return to userland.
+ */
+void fpregs_assert_state_consistent(void)
+{
+ struct fpu *fpu = &current->thread.fpu;
+
+ if (test_thread_flag(TIF_NEED_FPU_LOAD))
+ return;
+ WARN_ON_FPU(!fpregs_state_valid(fpu, smp_processor_id()));
+}
+EXPORT_SYMBOL_GPL(fpregs_assert_state_consistent);
+#endif
+
+void fpregs_mark_activate(void)
+{
+ struct fpu *fpu = &current->thread.fpu;
+
+ fpregs_activate(fpu);
+ fpu->last_cpu = smp_processor_id();
+ clear_thread_flag(TIF_NEED_FPU_LOAD);
+}
+EXPORT_SYMBOL_GPL(fpregs_mark_activate);
+
/*
* x87 math exception handling:
*/
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index a17e75fa1a0a6..61a03a34a7304 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -230,11 +230,9 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
struct fpu *fpu = &tsk->thread.fpu;
int state_size = fpu_kernel_xstate_size;
struct user_i387_ia32_struct env;
- union fpregs_state *state;
u64 xfeatures = 0;
int fx_only = 0;
int ret = 0;
- void *tmp;

ia32_fxstate &= (IS_ENABLED(CONFIG_X86_32) ||
IS_ENABLED(CONFIG_IA32_EMULATION));
@@ -269,14 +267,18 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
}
}

- tmp = kzalloc(sizeof(*state) + fpu_kernel_xstate_size + 64, GFP_KERNEL);
- if (!tmp)
- return -ENOMEM;
- state = PTR_ALIGN(tmp, 64);
+ /*
+ * The current state of the FPU registers does not matter. By setting
+ * TIF_NEED_FPU_LOAD unconditionally it is ensured that the our xstate
+ * is not modified on context switch and that the xstate is considered
+ * to loaded again on return to userland (overriding last_cpu avoids the
+ * optimisation).
+ */
+ set_thread_flag(TIF_NEED_FPU_LOAD);
+ __fpu_invalidate_fpregs_state(fpu);

if ((unsigned long)buf_fx % 64)
fx_only = 1;
-
/*
* For 32-bit frames with fxstate, copy the fxstate so it can be
* reconstructed later.
@@ -292,43 +294,51 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
u64 init_bv = xfeatures_mask & ~xfeatures;

if (using_compacted_format()) {
- ret = copy_user_to_xstate(&state->xsave, buf_fx);
+ ret = copy_user_to_xstate(&fpu->state.xsave, buf_fx);
} else {
- ret = __copy_from_user(&state->xsave, buf_fx, state_size);
+ ret = __copy_from_user(&fpu->state.xsave, buf_fx, state_size);

if (!ret && state_size > offsetof(struct xregs_state, header))
- ret = validate_xstate_header(&state->xsave.header);
+ ret = validate_xstate_header(&fpu->state.xsave.header);
}
if (ret)
goto err_out;

- sanitize_restored_xstate(state, envp, xfeatures, fx_only);
+ sanitize_restored_xstate(&fpu->state, envp, xfeatures, fx_only);

+ __fpregs_changes_begin();
if (unlikely(init_bv))
copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
- ret = copy_users_to_xregs(&state->xsave, xfeatures);
+ ret = copy_users_to_xregs(&fpu->state.xsave, xfeatures);

} else if (use_fxsr()) {
- ret = __copy_from_user(&state->fxsave, buf_fx, state_size);
- if (ret)
+ ret = __copy_from_user(&fpu->state.fxsave, buf_fx, state_size);
+ if (ret) {
+ ret = -EFAULT;
goto err_out;
+ }

- sanitize_restored_xstate(state, envp, xfeatures, fx_only);
+ sanitize_restored_xstate(&fpu->state, envp, xfeatures, fx_only);
+
+ __fpregs_changes_begin();
if (use_xsave()) {
u64 init_bv = xfeatures_mask & ~XFEATURE_MASK_FPSSE;
copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
}

- ret = copy_users_to_fxregs(&state->fxsave);
+ ret = copy_users_to_fxregs(&fpu->state.fxsave);
} else {
- ret = __copy_from_user(&state->fsave, buf_fx, state_size);
+ ret = __copy_from_user(&fpu->state.fsave, buf_fx, state_size);
if (ret)
goto err_out;
+ __fpregs_changes_begin();
ret = copy_users_to_fregs(buf_fx);
}
+ if (!ret)
+ fpregs_mark_activate();
+ __fpregs_changes_end();

err_out:
- kfree(tmp);
if (ret)
fpu__clear(fpu);
return ret;
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 90ae0ca510837..2e38a14fdbd3f 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -101,7 +101,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
dst->thread.vm86 = NULL;
#endif

- return fpu__copy(&dst->thread.fpu, &src->thread.fpu);
+ return fpu__copy(dst, src);
}

/*
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 77d9eb43ccac8..1bc47f3a48854 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -234,7 +234,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)

/* never put a printk in __switch_to... printk() calls wake_up*() indirectly */

- switch_fpu_prepare(prev_fpu, cpu);
+ if (!test_thread_flag(TIF_NEED_FPU_LOAD))
+ switch_fpu_prepare(prev_fpu, cpu);

/*
* Save away %gs. No need to save %fs, as it was saved on the
@@ -290,7 +291,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)

this_cpu_write(current_task, next_p);

- switch_fpu_finish(next_fpu, cpu);
+ switch_fpu_finish(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 ffea7c557963a..37b2ecef041e6 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -520,7 +520,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ENTRY) &&
this_cpu_read(irq_count) != -1);

- switch_fpu_prepare(prev_fpu, cpu);
+ if (!test_thread_flag(TIF_NEED_FPU_LOAD))
+ switch_fpu_prepare(prev_fpu, cpu);

/* We must save %fs and %gs before load_TLS() because
* %fs and %gs may be cleared by load_TLS().
@@ -572,7 +573,7 @@ __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, cpu);
+ switch_fpu_finish(next_fpu);

/* Reload sp0. */
update_task_stack(next_p);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6c21aa5c00e58..e52e8ac73e755 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7833,6 +7833,10 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
wait_lapic_expire(vcpu);
guest_enter_irqoff();

+ fpregs_assert_state_consistent();
+ if (test_thread_flag(TIF_NEED_FPU_LOAD))
+ switch_fpu_return();
+
if (unlikely(vcpu->arch.switch_db_regs)) {
set_debugreg(0, 7);
set_debugreg(vcpu->arch.eff_db[0], 0);
@@ -8092,22 +8096,30 @@ static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
/* Swap (qemu) user FPU context for the guest FPU context. */
static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
{
- preempt_disable();
+ __fpregs_changes_begin();
+
copy_fpregs_to_fpstate(&current->thread.fpu);
/* PKRU is separately restored in kvm_x86_ops->run. */
__copy_kernel_to_fpregs(&vcpu->arch.guest_fpu->state,
~XFEATURE_MASK_PKRU);
- preempt_enable();
+
+ fpregs_mark_activate();
+ __fpregs_changes_end();
+
trace_kvm_fpu(1);
}

/* When vcpu_run ends, restore user space FPU context. */
static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
{
- preempt_disable();
+ __fpregs_changes_begin();
+
copy_fpregs_to_fpstate(vcpu->arch.guest_fpu);
copy_kernel_to_fpregs(&current->thread.fpu.state);
- preempt_enable();
+
+ fpregs_mark_activate();
+ __fpregs_changes_end();
+
++vcpu->stat.fpu_reload;
trace_kvm_fpu(0);
}
--
2.20.1