Re: [Qemu-devel] [RFC v7 13/16] softmmu: Add history of excl accesses

[alvise rigo]

On Tue, Feb 16, 2016 at 6:07 PM, Alex Bennée <address@hidden> wrote:
>
> Alvise Rigo <address@hidden> writes:
>
>> Add a circular buffer to store the hw addresses used in the last
>> EXCLUSIVE_HISTORY_LEN exclusive accesses.
>>
>> When an address is pop'ed from the buffer, its page will be set as not
>> exclusive. In this way, we avoid:
>> - frequent set/unset of a page (causing frequent flushes as well)
>> - the possibility to forget the EXCL bit set.
>
> Why was this a possibility before? Shouldn't that be tackled in the
> patch that introduced it?

Yes and no. The problem happens for instance when a LL will not be
followed by the SC. In this situation, the flag will be set for the
page, but might remain set for the rest of the execution (unless a
complete LL/SC in performed later on in the same guest page).

>
>>
>> Suggested-by: Jani Kokkonen <address@hidden>
>> Suggested-by: Claudio Fontana <address@hidden>
>> Signed-off-by: Alvise Rigo <address@hidden>
>> ---
>>  cputlb.c                | 29 +++++++++++++++++++----------
>>  exec.c                  | 19 +++++++++++++++++++
>>  include/qom/cpu.h       |  8 ++++++++
>>  softmmu_llsc_template.h |  1 +
>>  vl.c                    |  3 +++
>>  5 files changed, 50 insertions(+), 10 deletions(-)
>>
>> diff --git a/cputlb.c b/cputlb.c
>> index 06ce2da..f3c4d97 100644
>> --- a/cputlb.c
>> +++ b/cputlb.c
>> @@ -395,16 +395,6 @@ void tlb_set_page_with_attrs(CPUState *cpu, 
>> target_ulong vaddr,
>>      env->tlb_v_table[mmu_idx][vidx] = *te;
>>      env->iotlb_v[mmu_idx][vidx] = env->iotlb[mmu_idx][index];
>>
>> -    if (unlikely(!(te->addr_write & TLB_MMIO) && (te->addr_write & 
>> TLB_EXCL))) {
>> -        /* We are removing an exclusive entry, set the page to dirty. This
>> -         * is not be necessary if the vCPU has performed both SC and LL. */
>> -        hwaddr hw_addr = (env->iotlb[mmu_idx][index].addr & 
>> TARGET_PAGE_MASK) +
>> -                                          (te->addr_write & 
>> TARGET_PAGE_MASK);
>> -        if (!cpu->ll_sc_context) {
>> -            cpu_physical_memory_unset_excl(hw_addr);
>> -        }
>> -    }
>> -
>
> Errm is this right? I got confused reviewing 8/16 because my final tree
> didn't have this code. I'm not sure the adding of history obviates the
> need to clear the exclusive flag?

We will clear it adding a new item to the history. When an entry is
added, the oldest is removed and cleaned, solving the problem
mentioned above.

Thank you,
alvise

>
>>      /* refill the tlb */
>>      env->iotlb[mmu_idx][index].addr = iotlb - vaddr;
>>      env->iotlb[mmu_idx][index].attrs = attrs;
>> @@ -517,6 +507,25 @@ static inline bool lookup_and_reset_cpus_ll_addr(hwaddr 
>> addr, hwaddr size)
>>      return ret;
>>  }
>>
>> +extern CPUExclusiveHistory excl_history;
>> +static inline void excl_history_put_addr(hwaddr addr)
>> +{
>> +    hwaddr last;
>> +
>> +    /* Calculate the index of the next exclusive address */
>> +    excl_history.last_idx = (excl_history.last_idx + 1) % 
>> excl_history.length;
>> +
>> +    last = excl_history.c_array[excl_history.last_idx];
>> +
>> +    /* Unset EXCL bit of the oldest entry */
>> +    if (last != EXCLUSIVE_RESET_ADDR) {
>> +        cpu_physical_memory_unset_excl(last);
>> +    }
>> +
>> +    /* Add a new address, overwriting the oldest one */
>> +    excl_history.c_array[excl_history.last_idx] = addr & TARGET_PAGE_MASK;
>> +}
>> +
>>  #define MMUSUFFIX _mmu
>>
>>  /* Generates LoadLink/StoreConditional helpers in softmmu_template.h */
>> diff --git a/exec.c b/exec.c
>> index 51f366d..2e123f1 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -177,6 +177,25 @@ struct CPUAddressSpace {
>>      MemoryListener tcg_as_listener;
>>  };
>>
>> +/* Exclusive memory support */
>> +CPUExclusiveHistory excl_history;
>> +void cpu_exclusive_history_init(void)
>> +{
>> +    /* Initialize exclusive history for atomic instruction handling. */
>> +    if (tcg_enabled()) {
>> +        g_assert(EXCLUSIVE_HISTORY_CPU_LEN * max_cpus <= UINT16_MAX);
>> +        excl_history.length = EXCLUSIVE_HISTORY_CPU_LEN * max_cpus;
>> +        excl_history.c_array = g_malloc(excl_history.length * 
>> sizeof(hwaddr));
>> +        memset(excl_history.c_array, -1, excl_history.length * 
>> sizeof(hwaddr));
>> +    }
>> +}
>> +
>> +void cpu_exclusive_history_free(void)
>> +{
>> +    if (tcg_enabled()) {
>> +        g_free(excl_history.c_array);
>> +    }
>> +}
>>  #endif
>>
>>  #if !defined(CONFIG_USER_ONLY)
>> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
>> index 6f6c1c0..0452fd0 100644
>> --- a/include/qom/cpu.h
>> +++ b/include/qom/cpu.h
>> @@ -227,7 +227,15 @@ struct kvm_run;
>>  #define TB_JMP_CACHE_SIZE (1 << TB_JMP_CACHE_BITS)
>>
>>  /* Atomic insn translation TLB support. */
>> +typedef struct CPUExclusiveHistory {
>> +    uint16_t last_idx;           /* index of last insertion */
>> +    uint16_t length;             /* history's length, it depends on 
>> smp_cpus */
>> +    hwaddr *c_array;             /* history's circular array */
>> +} CPUExclusiveHistory;
>>  #define EXCLUSIVE_RESET_ADDR ULLONG_MAX
>> +#define EXCLUSIVE_HISTORY_CPU_LEN 256
>> +void cpu_exclusive_history_init(void);
>> +void cpu_exclusive_history_free(void);
>>
>>  /**
>>   * CPUState:
>> diff --git a/softmmu_llsc_template.h b/softmmu_llsc_template.h
>> index b4712ba..b4e7f9d 100644
>> --- a/softmmu_llsc_template.h
>> +++ b/softmmu_llsc_template.h
>> @@ -75,6 +75,7 @@ WORD_TYPE helper_ldlink_name(CPUArchState *env, 
>> target_ulong addr,
>>           * to request any flush. */
>>          if (!cpu_physical_memory_is_excl(hw_addr)) {
>>              cpu_physical_memory_set_excl(hw_addr);
>> +            excl_history_put_addr(hw_addr);
>>              CPU_FOREACH(cpu) {
>>                  if (current_cpu != cpu) {
>>                      tlb_flush(cpu, 1);
>> diff --git a/vl.c b/vl.c
>> index f043009..b22d99b 100644
>> --- a/vl.c
>> +++ b/vl.c
>> @@ -547,6 +547,7 @@ static void res_free(void)
>>  {
>>      g_free(boot_splash_filedata);
>>      boot_splash_filedata = NULL;
>> +    cpu_exclusive_history_free();
>>  }
>>
>>  static int default_driver_check(void *opaque, QemuOpts *opts, Error **errp)
>> @@ -4322,6 +4323,8 @@ int main(int argc, char **argv, char **envp)
>>
>>      configure_accelerator(current_machine);
>>
>> +    cpu_exclusive_history_init();
>> +
>>      if (qtest_chrdev) {
>>          qtest_init(qtest_chrdev, qtest_log, &error_fatal);
>>      }
>
>
> --
> Alex Bennée