Currently the FPSIMD handling code uses the condition task->mm ==
NULL as a hint that task has no FPSIMD register context.
The ->mm check is only there to filter out tasks that cannot
possibly have FPSIMD context loaded, for optimisation purposes.
Also, TIF_FOREIGN_FPSTATE must always be checked anyway before
saving FPSIMD context back to memory. For these reasons, the ->mm
checks are not useful, providing that that TIF_FOREIGN_FPSTATE is
maintained in a consistent way for kernel threads.
This is true by construction however: TIF_FOREIGN_FPSTATE is never
cleared except when returning to userspace or returning from a
signal: thus, for a true kernel thread no FPSIMD context is ever
loaded, TIF_FOREIGN_FPSTATE will remain set and no context will
ever be saved.
This patch removes the redundant checks and special-case code.
Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
---
Changes since v9:
* New patch. Introduced during debugging, since the ->mm checks
appear bogus and/or redundant, so are likely to be hiding or
causing bugs.
---
arch/arm64/include/asm/thread_info.h | 1 +
arch/arm64/kernel/fpsimd.c | 38 ++++++++++++------------------------
2 files changed, 14 insertions(+), 25 deletions(-)
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index 740aa03c..a2ac914 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -47,6 +47,7 @@ struct thread_info {
#define INIT_THREAD_INFO(tsk) \
{ \
+ .flags = _TIF_FOREIGN_FPSTATE, \
.preempt_count = INIT_PREEMPT_COUNT, \
.addr_limit = KERNEL_DS, \
}
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 3aa100a..1222491 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -891,31 +891,21 @@ asmlinkage void do_fpsimd_exc(unsigned int esr, struct pt_regs *regs)
void fpsimd_thread_switch(struct task_struct *next)
{
+ bool wrong_task, wrong_cpu;
+
if (!system_supports_fpsimd())
return;
- /*
- * 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)
- fpsimd_save();
- 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.
- */
- bool wrong_task = __this_cpu_read(fpsimd_last_state.st) !=
+ /* Save unsaved fpsimd state, if any: */
+ fpsimd_save();
+
+ /* Fix up TIF_FOREIGN_FPSTATE to correctly describe next's state: */
+ wrong_task = __this_cpu_read(fpsimd_last_state.st) !=
&next->thread.uw.fpsimd_state;
- bool wrong_cpu = next->thread.fpsimd_cpu != smp_processor_id();
+ wrong_cpu = next->thread.fpsimd_cpu != smp_processor_id();
- update_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE,
- wrong_task || wrong_cpu);
- }
+ update_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE,
+ wrong_task || wrong_cpu);
}
void fpsimd_flush_thread(void)
@@ -1120,9 +1110,8 @@ void kernel_neon_begin(void)
__this_cpu_write(kernel_neon_busy, true);
- /* Save unsaved task fpsimd state, if any: */
- if (current->mm)
- fpsimd_save();
+ /* Save unsaved fpsimd state, if any: */
+ fpsimd_save();
/* Invalidate any task state remaining in the fpsimd regs: */
fpsimd_flush_cpu_state();
@@ -1244,8 +1233,7 @@ static int fpsimd_cpu_pm_notifier(struct notifier_block *self,
{
switch (cmd) {
case CPU_PM_ENTER:
- if (current->mm)
- fpsimd_save();
+ fpsimd_save();
fpsimd_flush_cpu_state();
break;
case CPU_PM_EXIT:
--
2.1.4
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