The following commit has been merged into the x86/percpu branch of tip:
Commit-ID: 24b8a23638cbf92449c353f828b1d309548c78f4
Gitweb: https://git.kernel.org/tip/24b8a23638cbf92449c353f828b1d309548c78f4
Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
AuthorDate: Wed, 18 Oct 2023 20:41:58 +02:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 20 Oct 2023 11:24:22 +02:00
x86/fpu: Clean up FPU switching in the middle of task switching
It happens to work, but it's very very wrong, because our 'current'
macro is magic that is supposedly loading a stable value.
It just happens to be not quite stable enough and the compilers
re-load the value enough for this code to work. But it's wrong.
The whole
struct fpu *prev_fpu = &prev->fpu;
thing in __switch_to() is pretty ugly. There's no reason why we
should look at that 'prev_fpu' pointer there, or pass it down.
And it only generates worse code, in how it loads 'current' when
__switch_to() has the right task pointers.
The attached patch not only cleans this up, it actually
generates better code too:
(a) it removes one push/pop pair at entry/exit because there's one
less register used (no 'current')
(b) it removes that pointless load of 'current' because it just uses
the right argument:
- movq %gs:pcpu_hot(%rip), %r12
- testq $16384, (%r12)
+ testq $16384, (%rdi)
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20231018184227.446318-1-ubizjak@xxxxxxxxx
---
arch/x86/include/asm/fpu/sched.h | 10 ++++++----
arch/x86/kernel/process_32.c | 7 +++----
arch/x86/kernel/process_64.c | 7 +++----
3 files changed, 12 insertions(+), 12 deletions(-)
diff --git a/arch/x86/include/asm/fpu/sched.h b/arch/x86/include/asm/fpu/sched.h
index ca6e5e5..c485f19 100644
--- a/arch/x86/include/asm/fpu/sched.h
+++ b/arch/x86/include/asm/fpu/sched.h
@@ -37,10 +37,12 @@ extern void fpu_flush_thread(void);
* The FPU context is only stored/restored for a user task and
* PF_KTHREAD is used to distinguish between kernel and user threads.
*/
-static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
+static inline void switch_fpu_prepare(struct task_struct *old, int cpu)
{
if (cpu_feature_enabled(X86_FEATURE_FPU) &&
- !(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
+ !(old->flags & (PF_KTHREAD | PF_USER_WORKER))) {
+ struct fpu *old_fpu = &old->thread.fpu;
+
save_fpregs_to_fpstate(old_fpu);
/*
* The save operation preserved register state, so the
@@ -60,10 +62,10 @@ static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
* Delay loading of the complete FPU state until the return to userland.
* PKRU is handled separately.
*/
-static inline void switch_fpu_finish(void)
+static inline void switch_fpu_finish(struct task_struct *new)
{
if (cpu_feature_enabled(X86_FEATURE_FPU))
- set_thread_flag(TIF_NEED_FPU_LOAD);
+ set_tsk_thread_flag(new, TIF_NEED_FPU_LOAD);
}
#endif /* _ASM_X86_FPU_SCHED_H */
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 708c87b..0917c7f 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -156,13 +156,12 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
{
struct thread_struct *prev = &prev_p->thread,
*next = &next_p->thread;
- struct fpu *prev_fpu = &prev->fpu;
int cpu = smp_processor_id();
/* never put a printk in __switch_to... printk() calls wake_up*() indirectly */
- if (!test_thread_flag(TIF_NEED_FPU_LOAD))
- switch_fpu_prepare(prev_fpu, cpu);
+ if (!test_tsk_thread_flag(prev_p, TIF_NEED_FPU_LOAD))
+ switch_fpu_prepare(prev_p, cpu);
/*
* Save away %gs. No need to save %fs, as it was saved on the
@@ -209,7 +208,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
raw_cpu_write(pcpu_hot.current_task, next_p);
- switch_fpu_finish();
+ switch_fpu_finish(next_p);
/* Load the Intel cache allocation PQR MSR. */
resctrl_sched_in(next_p);
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 33b2687..1553e19 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -562,14 +562,13 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
{
struct thread_struct *prev = &prev_p->thread;
struct thread_struct *next = &next_p->thread;
- struct fpu *prev_fpu = &prev->fpu;
int cpu = smp_processor_id();
WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ENTRY) &&
this_cpu_read(pcpu_hot.hardirq_stack_inuse));
- if (!test_thread_flag(TIF_NEED_FPU_LOAD))
- switch_fpu_prepare(prev_fpu, cpu);
+ if (!test_tsk_thread_flag(prev_p, TIF_NEED_FPU_LOAD))
+ switch_fpu_prepare(prev_p, cpu);
/* We must save %fs and %gs before load_TLS() because
* %fs and %gs may be cleared by load_TLS().
@@ -623,7 +622,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
raw_cpu_write(pcpu_hot.current_task, next_p);
raw_cpu_write(pcpu_hot.top_of_stack, task_top_of_stack(next_p));
- switch_fpu_finish();
+ switch_fpu_finish(next_p);
/* Reload sp0. */
update_task_stack(next_p);
|