[PATCH] riscv: vector: treat VS_INITIAL as discard
Andy Chiu
andy.chiu at sifive.com
Fri Jun 28 04:49:12 PDT 2024
The purpose of riscv_v_vstate_discard() is to invalidate v context at
entries of syscalls. So users happen to use v after a syscall without
re-configuring would see a failure. It was achieved by setting vector
registers and CSRs to -1 and marking the context busy. However, this
results in redundant saving of v-context if the process is scheduled out
in a syscall. Moreover, restoring the invalidated context from memory is
a costly operation. In fact, all can be prevented if we can delay
vstate_discard before returning back to the user space. To be more
specific, the kernel can mark v-context as INITIAL and set the restore
flag at syscall entries. This is the indication for the vstate_restore,
so it awares that the vstate has to be invalidated before returning back
to the user space.
After applying this patch, the context switch performance has improved
6.78% on vector enabled lmbench running on a FPGA with VLEN=512. The
result was obtained by averaging the output from the following command.
$ lat_ctx 2
Before the patch: 599.8357692
After the patch: 559.1748148
Signed-off-by: Andy Chiu <andy.chiu at sifive.com>
---
arch/riscv/include/asm/vector.h | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
index be7d309cca8a..1221f8b8f564 100644
--- a/arch/riscv/include/asm/vector.h
+++ b/arch/riscv/include/asm/vector.h
@@ -170,8 +170,8 @@ static inline void riscv_v_vstate_discard(struct pt_regs *regs)
if ((regs->status & SR_VS) == SR_VS_OFF)
return;
- __riscv_v_vstate_discard();
- __riscv_v_vstate_dirty(regs);
+ set_tsk_thread_flag(current, TIF_RISCV_V_DEFER_RESTORE);
+ riscv_v_vstate_on(regs);
}
static inline void riscv_v_vstate_save(struct __riscv_v_ext_state *vstate,
@@ -186,7 +186,9 @@ static inline void riscv_v_vstate_save(struct __riscv_v_ext_state *vstate,
static inline void riscv_v_vstate_restore(struct __riscv_v_ext_state *vstate,
struct pt_regs *regs)
{
- if ((regs->status & SR_VS) != SR_VS_OFF) {
+ if ((regs->status & SR_VS) == SR_VS_INITIAL) {
+ __riscv_v_vstate_discard();
+ } else if ((regs->status & SR_VS) != SR_VS_OFF) {
__riscv_v_vstate_restore(vstate, vstate->datap);
__riscv_v_vstate_clean(regs);
}
@@ -197,7 +199,7 @@ static inline void riscv_v_vstate_set_restore(struct task_struct *task,
{
if ((regs->status & SR_VS) != SR_VS_OFF) {
set_tsk_thread_flag(task, TIF_RISCV_V_DEFER_RESTORE);
- riscv_v_vstate_on(regs);
+ __riscv_v_vstate_clean(regs);
}
}
---
base-commit: 60a6707f582ebbdfb6b378f45d7bf929106a1cd5
change-id: 20240628-dev-vstate_discard-5784652968a3
Best regards,
--
Andy Chiu <andy.chiu at sifive.com>
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