#define FPEXC_IXF (1 << 4)
#define FPEXC_IDF (1 << 7)
+/*
+ * In order to reduce the number of times the FPSIMD state is needlessly saved
+ * and restored, we need to keep track of two things:
+ * (a) for each task, we need to remember which CPU was the last one to have
+ * the task's FPSIMD state loaded into its FPSIMD registers;
+ * (b) for each CPU, we need to remember which task's userland FPSIMD state has
+ * been loaded into its FPSIMD registers most recently, or whether it has
+ * been used to perform kernel mode NEON in the meantime.
+ *
+ * For (a), we add a 'cpu' field to struct fpsimd_state, which gets updated to
+ * the id of the current CPU everytime the state is loaded onto a CPU. For (b),
+ * we add the per-cpu variable 'fpsimd_last_state' (below), which contains the
+ * address of the userland FPSIMD state of the task that was loaded onto the CPU
+ * the most recently, or NULL if kernel mode NEON has been performed after that.
+ *
+ * With this in place, we no longer have to restore the next FPSIMD state right
+ * when switching between tasks. Instead, we can defer this check to userland
+ * resume, at which time we verify whether the CPU's fpsimd_last_state and the
+ * task's fpsimd_state.cpu are still mutually in sync. If this is the case, we
+ * can omit the FPSIMD restore.
+ *
+ * As an optimization, we use the thread_info flag TIF_FOREIGN_FPSTATE to
+ * indicate whether or not the userland FPSIMD state of the current task is
+ * present in the registers. The flag is set unless the FPSIMD registers of this
+ * CPU currently contain the most recent userland FPSIMD state of the current
+ * task.
+ *
+ * For a certain task, the sequence may look something like this:
+ * - the task gets scheduled in; if both the task's fpsimd_state.cpu field
+ * contains the id of the current CPU, and the CPU's fpsimd_last_state per-cpu
+ * variable points to the task's fpsimd_state, the TIF_FOREIGN_FPSTATE flag is
+ * cleared, otherwise it is set;
+ *
+ * - the task returns to userland; if TIF_FOREIGN_FPSTATE is set, the task's
+ * userland FPSIMD state is copied from memory to the registers, the task's
+ * fpsimd_state.cpu field is set to the id of the current CPU, the current
+ * CPU's fpsimd_last_state pointer is set to this task's fpsimd_state and the
+ * TIF_FOREIGN_FPSTATE flag is cleared;
+ *
+ * - the task executes an ordinary syscall; upon return to userland, the
+ * TIF_FOREIGN_FPSTATE flag will still be cleared, so no FPSIMD state is
+ * restored;
+ *
+ * - the task executes a syscall which executes some NEON instructions; this is
+ * preceded by a call to kernel_neon_begin(), which copies the task's FPSIMD
+ * register contents to memory, clears the fpsimd_last_state per-cpu variable
+ * and sets the TIF_FOREIGN_FPSTATE flag;
+ *
+ * - the task gets preempted after kernel_neon_end() is called; as we have not
+ * returned from the 2nd syscall yet, TIF_FOREIGN_FPSTATE is still set so
+ * whatever is in the FPSIMD registers is not saved to memory, but discarded.
+ */
+static DEFINE_PER_CPU(struct fpsimd_state *, fpsimd_last_state);
+
/*
* Trapped FP/ASIMD access.
*/
void fpsimd_thread_switch(struct task_struct *next)
{
- /* check if not kernel threads */
- if (current->mm)
+ /*
+ * Save the current FPSIMD state to memory, but only if whatever is in
+ * the registers is in fact the most recent userland FPSIMD state of
+ * 'current'.
+ */
+ if (current->mm && !test_thread_flag(TIF_FOREIGN_FPSTATE))
fpsimd_save_state(¤t->thread.fpsimd_state);
- if (next->mm)
- fpsimd_load_state(&next->thread.fpsimd_state);
+
+ if (next->mm) {
+ /*
+ * If we are switching to a task whose most recent userland
+ * FPSIMD state is already in the registers of *this* cpu,
+ * we can skip loading the state from memory. Otherwise, set
+ * the TIF_FOREIGN_FPSTATE flag so the state will be loaded
+ * upon the next return to userland.
+ */
+ struct fpsimd_state *st = &next->thread.fpsimd_state;
+
+ if (__this_cpu_read(fpsimd_last_state) == st
+ && st->cpu == smp_processor_id())
+ clear_ti_thread_flag(task_thread_info(next),
+ TIF_FOREIGN_FPSTATE);
+ else
+ set_ti_thread_flag(task_thread_info(next),
+ TIF_FOREIGN_FPSTATE);
+ }
}
void fpsimd_flush_thread(void)
{
- preempt_disable();
memset(¤t->thread.fpsimd_state, 0, sizeof(struct fpsimd_state));
- fpsimd_load_state(¤t->thread.fpsimd_state);
- preempt_enable();
+ set_thread_flag(TIF_FOREIGN_FPSTATE);
}
/*
- * Save the userland FPSIMD state of 'current' to memory
+ * Save the userland FPSIMD state of 'current' to memory, but only if the state
+ * currently held in the registers does in fact belong to 'current'
*/
void fpsimd_preserve_current_state(void)
{
preempt_disable();
- fpsimd_save_state(¤t->thread.fpsimd_state);
+ if (!test_thread_flag(TIF_FOREIGN_FPSTATE))
+ fpsimd_save_state(¤t->thread.fpsimd_state);
preempt_enable();
}
/*
- * Load an updated userland FPSIMD state for 'current' from memory
+ * Load the userland FPSIMD state of 'current' from memory, but only if the
+ * FPSIMD state already held in the registers is /not/ the most recent FPSIMD
+ * state of 'current'
+ */
+void fpsimd_restore_current_state(void)
+{
+ preempt_disable();
+ if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
+ struct fpsimd_state *st = ¤t->thread.fpsimd_state;
+
+ fpsimd_load_state(st);
+ this_cpu_write(fpsimd_last_state, st);
+ st->cpu = smp_processor_id();
+ }
+ preempt_enable();
+}
+
+/*
+ * Load an updated userland FPSIMD state for 'current' from memory and set the
+ * flag that indicates that the FPSIMD register contents are the most recent
+ * FPSIMD state of 'current'
*/
void fpsimd_update_current_state(struct fpsimd_state *state)
{
preempt_disable();
fpsimd_load_state(state);
+ if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
+ struct fpsimd_state *st = ¤t->thread.fpsimd_state;
+
+ this_cpu_write(fpsimd_last_state, st);
+ st->cpu = smp_processor_id();
+ }
preempt_enable();
}
+/*
+ * Invalidate live CPU copies of task t's FPSIMD state
+ */
+void fpsimd_flush_task_state(struct task_struct *t)
+{
+ t->thread.fpsimd_state.cpu = NR_CPUS;
+}
+
#ifdef CONFIG_KERNEL_MODE_NEON
/*
BUG_ON(in_interrupt());
preempt_disable();
- if (current->mm)
+ /*
+ * Save the userland FPSIMD state if we have one and if we haven't done
+ * so already. Clear fpsimd_last_state to indicate that there is no
+ * longer userland FPSIMD state in the registers.
+ */
+ if (current->mm && !test_and_set_thread_flag(TIF_FOREIGN_FPSTATE))
fpsimd_save_state(¤t->thread.fpsimd_state);
+ this_cpu_write(fpsimd_last_state, NULL);
}
EXPORT_SYMBOL(kernel_neon_begin);
void kernel_neon_end(void)
{
- if (current->mm)
- fpsimd_load_state(¤t->thread.fpsimd_state);
-
preempt_enable();
}
EXPORT_SYMBOL(kernel_neon_end);
{
switch (cmd) {
case CPU_PM_ENTER:
- if (current->mm)
+ if (current->mm && !test_thread_flag(TIF_FOREIGN_FPSTATE))
fpsimd_save_state(¤t->thread.fpsimd_state);
break;
case CPU_PM_EXIT:
if (current->mm)
- fpsimd_load_state(¤t->thread.fpsimd_state);
+ set_thread_flag(TIF_FOREIGN_FPSTATE);
break;
case CPU_PM_ENTER_FAILED:
default:
projects / linux.git / commitdiff