[PATCH 5/9] powerpc: Restore FPU/VEC/VSX if previously used
Cyril Bur
cyrilbur at gmail.com
Mon Feb 29 17:53:47 AEDT 2016
Currently the FPU, VEC and VSX facilities are lazily loaded. This is not a
problem unless a process is using these facilities.
Modern versions of GCC are very good at automatically vectorising code, new
and modernised workloads make use of floating point and vector facilities,
even the kernel makes use of vectorised memcpy.
All this combined greatly increases the cost of a syscall since the kernel
uses the facilities sometimes even in syscall fast-path making it
increasingly common for a thread to take an *_unavailable exception soon
after a syscall, not to mention potentially taking all three.
The obvious overcompensation to this problem is to simply always load all
the facilities on every exit to userspace. Loading up all FPU, VEC and VSX
registers every time can be expensive and if a workload does avoid using
them, it should not be forced to incur this penalty.
An 8bit counter is used to detect if the registers have been used in the
past and the registers are always loaded until the value wraps to back to
zero.
Several versions of the assembly in entry_64.S. 1. Always calling C, 2.
Performing a common case check and then calling C and 3. A complex check in
asm. After some benchmarking it was determined that avoiding C in the
common case is a performance benefit. The full check in asm greatly
complicated that codepath for a negligible performance gain and the
trade-off was deemed not worth it.
Signed-off-by: Cyril Bur <cyrilbur at gmail.com>
---
arch/powerpc/include/asm/processor.h | 2 +
arch/powerpc/kernel/asm-offsets.c | 2 +
arch/powerpc/kernel/entry_64.S | 21 +++++++--
arch/powerpc/kernel/fpu.S | 4 ++
arch/powerpc/kernel/process.c | 88 +++++++++++++++++++++++++++++++-----
arch/powerpc/kernel/vector.S | 4 ++
6 files changed, 107 insertions(+), 14 deletions(-)
diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index ac23308..dcab21f 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -236,11 +236,13 @@ struct thread_struct {
#endif
struct arch_hw_breakpoint hw_brk; /* info on the hardware breakpoint */
unsigned long trap_nr; /* last trap # on this thread */
+ u8 load_fp;
#ifdef CONFIG_ALTIVEC
struct thread_vr_state vr_state;
struct thread_vr_state *vr_save_area;
unsigned long vrsave;
int used_vr; /* set if process has used altivec */
+ u8 load_vec;
#endif /* CONFIG_ALTIVEC */
#ifdef CONFIG_VSX
/* VSR status */
diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index 07cebc3..10d5eab 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -95,12 +95,14 @@ int main(void)
DEFINE(THREAD_FPSTATE, offsetof(struct thread_struct, fp_state));
DEFINE(THREAD_FPSAVEAREA, offsetof(struct thread_struct, fp_save_area));
DEFINE(FPSTATE_FPSCR, offsetof(struct thread_fp_state, fpscr));
+ DEFINE(THREAD_LOAD_FP, offsetof(struct thread_struct, load_fp));
#ifdef CONFIG_ALTIVEC
DEFINE(THREAD_VRSTATE, offsetof(struct thread_struct, vr_state));
DEFINE(THREAD_VRSAVEAREA, offsetof(struct thread_struct, vr_save_area));
DEFINE(THREAD_VRSAVE, offsetof(struct thread_struct, vrsave));
DEFINE(THREAD_USED_VR, offsetof(struct thread_struct, used_vr));
DEFINE(VRSTATE_VSCR, offsetof(struct thread_vr_state, vscr));
+ DEFINE(THREAD_LOAD_VEC, offsetof(struct thread_struct, load_vec));
#endif /* CONFIG_ALTIVEC */
#ifdef CONFIG_VSX
DEFINE(THREAD_USED_VSR, offsetof(struct thread_struct, used_vsr));
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 0d525ce..038e0a1 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -210,7 +210,20 @@ system_call: /* label this so stack traces look sane */
li r11,-MAX_ERRNO
andi. r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|_TIF_PERSYSCALL_MASK)
bne- syscall_exit_work
- cmpld r3,r11
+
+ andi. r0,r8,MSR_FP
+ beq 2f
+#ifdef CONFIG_ALTIVEC
+ andis. r0,r8,MSR_VEC at h
+ bne 3f
+#endif
+2: addi r3,r1,STACK_FRAME_OVERHEAD
+ bl restore_math
+ ld r8,_MSR(r1)
+ ld r3,RESULT(r1)
+ li r11,-MAX_ERRNO
+
+3: cmpld r3,r11
ld r5,_CCR(r1)
bge- syscall_error
.Lsyscall_error_cont:
@@ -602,8 +615,8 @@ _GLOBAL(ret_from_except_lite)
/* Check current_thread_info()->flags */
andi. r0,r4,_TIF_USER_WORK_MASK
-#ifdef CONFIG_PPC_BOOK3E
bne 1f
+#ifdef CONFIG_PPC_BOOK3E
/*
* Check to see if the dbcr0 register is set up to debug.
* Use the internal debug mode bit to do this.
@@ -618,7 +631,9 @@ _GLOBAL(ret_from_except_lite)
mtspr SPRN_DBSR,r10
b restore
#else
- beq restore
+ addi r3,r1,STACK_FRAME_OVERHEAD
+ bl restore_math
+ b restore
#endif
1: andi. r0,r4,_TIF_NEED_RESCHED
beq 2f
diff --git a/arch/powerpc/kernel/fpu.S b/arch/powerpc/kernel/fpu.S
index 2117eac..b063524 100644
--- a/arch/powerpc/kernel/fpu.S
+++ b/arch/powerpc/kernel/fpu.S
@@ -130,6 +130,10 @@ END_FTR_SECTION_IFSET(CPU_FTR_VSX)
or r12,r12,r4
std r12,_MSR(r1)
#endif
+ /* Don't care if r4 overflows, this is desired behaviour */
+ lbz r4,THREAD_LOAD_FP(r5)
+ addi r4,r4,1
+ stb r4,THREAD_LOAD_FP(r5)
addi r10,r5,THREAD_FPSTATE
lfd fr0,FPSTATE_FPSCR(r10)
MTFSF_L(fr0)
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index e0c3d2d..55c1eb0 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -187,9 +187,22 @@ void enable_kernel_fp(void)
}
}
EXPORT_SYMBOL(enable_kernel_fp);
+
+static int restore_fp(struct task_struct *tsk) {
+ if (tsk->thread.load_fp) {
+ load_fp_state(¤t->thread.fp_state);
+ current->thread.load_fp++;
+ return 1;
+ }
+ return 0;
+}
+#else
+static int restore_fp(struct task_struct *tsk) { return 0; }
#endif /* CONFIG_PPC_FPU */
#ifdef CONFIG_ALTIVEC
+#define loadvec(thr) ((thr).load_vec)
+
void giveup_altivec(struct task_struct *tsk)
{
check_if_tm_restore_required(tsk);
@@ -229,6 +242,21 @@ void flush_altivec_to_thread(struct task_struct *tsk)
}
}
EXPORT_SYMBOL_GPL(flush_altivec_to_thread);
+
+static int restore_altivec(struct task_struct *tsk)
+{
+ if (cpu_has_feature(CPU_FTR_ALTIVEC) && tsk->thread.load_vec) {
+ load_vr_state(&tsk->thread.vr_state);
+ tsk->thread.used_vr = 1;
+ tsk->thread.load_vec++;
+
+ return 1;
+ }
+ return 0;
+}
+#else
+#define loadvec(thr) 0
+static inline int restore_altivec(struct task_struct *tsk) { return 0; }
#endif /* CONFIG_ALTIVEC */
#ifdef CONFIG_VSX
@@ -275,6 +303,18 @@ void flush_vsx_to_thread(struct task_struct *tsk)
}
}
EXPORT_SYMBOL_GPL(flush_vsx_to_thread);
+
+static int restore_vsx(struct task_struct *tsk)
+{
+ if (cpu_has_feature(CPU_FTR_VSX)) {
+ tsk->thread.used_vsr = 1;
+ return 1;
+ }
+
+ return 0;
+}
+#else
+static inline int restore_vsx(struct task_struct *tsk) { return 0; }
#endif /* CONFIG_VSX */
#ifdef CONFIG_SPE
@@ -374,6 +414,36 @@ void giveup_all(struct task_struct *tsk)
}
EXPORT_SYMBOL(giveup_all);
+void restore_math(struct pt_regs *regs)
+{
+ unsigned long msr;
+
+ if (!current->thread.load_fp && !loadvec(current->thread))
+ return;
+
+ msr = regs->msr;
+ msr_check_and_set(msr_all_available);
+
+ /*
+ * Only reload if the bit is not set in the user MSR, the bit BEING set
+ * indicates that the registers are hot
+ */
+ if ((!(msr & MSR_FP)) && restore_fp(current))
+ msr |= MSR_FP | current->thread.fpexc_mode;
+
+ if ((!(msr & MSR_VEC)) && restore_altivec(current))
+ msr |= MSR_VEC;
+
+ if ((msr & (MSR_FP | MSR_VEC)) == (MSR_FP | MSR_VEC) &&
+ restore_vsx(current)) {
+ msr |= MSR_VSX;
+ }
+
+ msr_check_and_clear(msr_all_available);
+
+ regs->msr = msr;
+}
+
void flush_all_to_thread(struct task_struct *tsk)
{
if (tsk->thread.regs) {
@@ -832,17 +902,9 @@ void restore_tm_state(struct pt_regs *regs)
msr_diff = current->thread.ckpt_regs.msr & ~regs->msr;
msr_diff &= MSR_FP | MSR_VEC | MSR_VSX;
- if (msr_diff & MSR_FP) {
- msr_check_and_set(MSR_FP);
- load_fp_state(¤t->thread.fp_state);
- msr_check_and_clear(MSR_FP);
- regs->msr |= current->thread.fpexc_mode;
- }
- if (msr_diff & MSR_VEC) {
- msr_check_and_set(MSR_VEC);
- load_vr_state(¤t->thread.vr_state);
- msr_check_and_clear(MSR_VEC);
- }
+
+ restore_math(regs);
+
regs->msr |= msr_diff;
}
@@ -1006,6 +1068,10 @@ struct task_struct *__switch_to(struct task_struct *prev,
batch = this_cpu_ptr(&ppc64_tlb_batch);
batch->active = 1;
}
+
+ if (current_thread_info()->task->thread.regs)
+ restore_math(current_thread_info()->task->thread.regs);
+
#endif /* CONFIG_PPC_BOOK3S_64 */
return last;
diff --git a/arch/powerpc/kernel/vector.S b/arch/powerpc/kernel/vector.S
index 162d0f7..038cff8 100644
--- a/arch/powerpc/kernel/vector.S
+++ b/arch/powerpc/kernel/vector.S
@@ -91,6 +91,10 @@ _GLOBAL(load_up_altivec)
oris r12,r12,MSR_VEC at h
std r12,_MSR(r1)
#endif
+ /* Don't care if r4 overflows, this is desired behaviour */
+ lbz r4,THREAD_LOAD_VEC(r5)
+ addi r4,r4,1
+ stb r4,THREAD_LOAD_VEC(r5)
addi r6,r5,THREAD_VRSTATE
li r4,1
li r10,VRSTATE_VSCR
--
2.7.2
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