From: Fei Wu <fei2.wu@intel.com>
Kernel needs to access user mode memory e.g. during syscalls, the window
is usually opened up for a very limited time through MSTATUS.SUM, the
overhead is too much if tlb_flush() gets called for every SUM change.
This patch saves addresses accessed when SUM=1, and flushs only these
pages when SUM changes to 0. If the buffer is not large enough to save
all the pages during SUM=1, it will fall back to tlb_flush when
necessary.
The buffer size is set to 4 since in this MSTATUS.SUM open-up window,
most of the time kernel accesses 1 or 2 pages, it's very rare to see
more than 4 pages accessed.
It's not necessary to save/restore these new added status, as
tlb_flush() is always called after restore.
Result of 'pipe 10' from unixbench boosts from 223656 to 1327407. Many
other syscalls benefit a lot from this one too.
Signed-off-by: Fei Wu <fei2.wu@intel.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
---
target/riscv/cpu.h | 4 ++++
target/riscv/cpu_helper.c | 7 +++++++
target/riscv/csr.c | 14 +++++++++++++-
3 files changed, 24 insertions(+), 1 deletion(-)
diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
index 638e47c75a..926dbce59f 100644
--- a/target/riscv/cpu.h
+++ b/target/riscv/cpu.h
@@ -383,6 +383,10 @@ struct CPUArchState {
uint64_t kvm_timer_compare;
uint64_t kvm_timer_state;
uint64_t kvm_timer_frequency;
+
+#define MAX_CACHED_SUM_U_ADDR_NUM 4
+ uint64_t sum_u_count;
+ uint64_t sum_u_addr[MAX_CACHED_SUM_U_ADDR_NUM];
};
OBJECT_DECLARE_CPU_TYPE(RISCVCPU, RISCVCPUClass, RISCV_CPU)
diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
index f88c503cf4..5ad0418eb6 100644
--- a/target/riscv/cpu_helper.c
+++ b/target/riscv/cpu_helper.c
@@ -1068,6 +1068,13 @@ restart:
(access_type == MMU_DATA_STORE || (pte & PTE_D))) {
*prot |= PAGE_WRITE;
}
+ if ((pte & PTE_U) && (mode & PRV_S) &&
+ get_field(env->mstatus, MSTATUS_SUM)) {
+ if (env->sum_u_count < MAX_CACHED_SUM_U_ADDR_NUM) {
+ env->sum_u_addr[env->sum_u_count] = addr;
+ }
+ ++env->sum_u_count;
+ }
return TRANSLATE_SUCCESS;
}
}
diff --git a/target/riscv/csr.c b/target/riscv/csr.c
index ab566639e5..74b7638c8a 100644
--- a/target/riscv/csr.c
+++ b/target/riscv/csr.c
@@ -1246,9 +1246,21 @@ static RISCVException
write_mstatus(CPURISCVState *env, int csrno,
/* flush tlb on mstatus fields that affect VM */
if ((val ^ mstatus) & (MSTATUS_MXR | MSTATUS_MPP | MSTATUS_MPV |
- MSTATUS_MPRV | MSTATUS_SUM)) {
+ MSTATUS_MPRV)) {
tlb_flush(env_cpu(env));
+ env->sum_u_count = 0;
+ } else if ((mstatus & MSTATUS_SUM) && !(val & MSTATUS_SUM)) {
+ if (env->sum_u_count > MAX_CACHED_SUM_U_ADDR_NUM) {
+ tlb_flush(env_cpu(env));