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

[Wu, Fei]

On 3/21/2023 4:28 PM, liweiwei wrote:
> 
> On 2023/3/21 14:37, fei2.wu@intel.com wrote:
>> 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));
>> +        } 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;
>>       }
> 
> Whether tlb should  be flushed when SUM is changed from 0 to 1?
> 
When SUM is changed from 0 to 1, all the existing tlb entries remain
valid as the permission is elevated instead of reduced, so I don't think
it's necessary to flush tlb.

Thanks,
Fei.

> Regards,
> 
> Weiwei Li
> 
>> +
>>       mask = MSTATUS_SIE | MSTATUS_SPIE | MSTATUS_MIE | MSTATUS_MPIE |
>>           MSTATUS_SPP | MSTATUS_MPRV | MSTATUS_SUM |
>>           MSTATUS_MPP | MSTATUS_MXR | MSTATUS_TVM | MSTATUS_TSR |
>