[PATCH v2] target/riscv: reduce overhead of MSTATUS_SUM change

[Fei Wu]

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...@intel.com>
Reviewed-by: LIU Zhiwei <zhiwei_...@linux.alibaba.com>
Reviewed-by: Weiwei Li <liwei...@iscas.ac.cn>
---
 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..d701017a60 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..e7dfdc6a93 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_by_mmuidx(env_cpu(env), 1 << PRV_S | 1 << PRV_M);
+        } else {
+            for (int i = 0; i < env->sum_u_count; ++i) {
+                tlb_flush_page_by_mmuidx(env_cpu(env), env->sum_u_addr[i],
+                                         1 << PRV_S | 1 << PRV_M);
+            }
+        }
+        env->sum_u_count = 0;
     }
+
     mask = MSTATUS_SIE | MSTATUS_SPIE | MSTATUS_MIE | MSTATUS_MPIE |
         MSTATUS_SPP | MSTATUS_MPRV | MSTATUS_SUM |
         MSTATUS_MPP | MSTATUS_MXR | MSTATUS_TVM | MSTATUS_TSR |
-- 
2.25.1