riscv: use vDSO common flow to reduce the latency of the time-related functions

Even if RISC-V has supported the vDSO feature, the latency of the functions
for obtaining the system time is still expensive. It is because these
functions still trigger a corresponding system call in the process, which
slows down the response time. If we want to remove the system call to
reduce the latency, the kernel should have the ability to output the system
clock information to userspace. This patch introduces the vDSO common flow
to enable the kernel to achieve the above feature and uses "rdtime"
instruction to obtain the current time in the user space. Under this
condition, the latency cost by the ecall from U-mode to S-mode can be
eliminated. After applying this patch, the latency of gettimeofday()
measured on the HiFive unleashed board can be reduced by %61.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

1 files changed, 2 insertions, 10 deletions

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index b1efd840003c..bdddcd5c1b71 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -8,6 +8,8 @@
 
 #include <linux/const.h>
 
+#include <vdso/processor.h>
+
 #include <asm/ptrace.h>
 
 /*
@@ -58,16 +60,6 @@ static inline void release_thread(struct task_struct *dead_task)
 extern unsigned long get_wchan(struct task_struct *p);
 
 
-static inline void cpu_relax(void)
-{
-#ifdef __riscv_muldiv
-	int dummy;
-	/* In lieu of a halt instruction, induce a long-latency stall. */
-	__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
-#endif
-	barrier();
-}
-
 static inline void wait_for_interrupt(void)
 {
 	__asm__ __volatile__ ("wfi");