Re: [Qemu-devel] [PATCH v3 07/11] tcg: Prepare TB invalidation for lockless TB lookup

[Sergey Fedorov]

On 14/07/16 15:59, Alex Bennée wrote:
> Sergey Fedorov <address@hidden> writes:
>
>> From: Sergey Fedorov <address@hidden>
>>
>> When invalidating a translation block, set an invalid CPU state into the
>> TranslationBlock structure first. All subsequent changes are ordered
>> after it with smp_wmb(). This pairs with implied smp_rmb() of
>> qht_lookup() in tb_find_physical().
>>
>> As soon as the TB is marked with an invalid CPU state, there is no need
>> to remove it from CPU's 'tb_jmp_cache'. However it will be necessary to
>> recheck whether the target TB is still valid after acquiring 'tb_lock'
>> but before calling tb_add_jump() since TB lookup is to be performed out
>> of 'tb_lock' in future. Note that we don't have to check 'last_tb' since
>> it is safe to patch an already invalidated TB since it will not be
>> executed anyway.
> This looks fine. So I guess it's possible for the block to have been
> looked up just before it gets invalidated and get one final run but not
> get patched?

Yes, exactly.

Thanks,
Sergey

>
> Reviewed-by: Alex Bennée <address@hidden>
>
>> Suggested-by: Paolo Bonzini <address@hidden>
>> Signed-off-by: Sergey Fedorov <address@hidden>
>> Signed-off-by: Sergey Fedorov <address@hidden>
>> ---
>>  cpu-exec.c      |  2 +-
>>  translate-all.c | 12 +++---------
>>  2 files changed, 4 insertions(+), 10 deletions(-)
>>
>> diff --git a/cpu-exec.c b/cpu-exec.c
>> index c973e3b85922..07dc50c56e8d 100644
>> --- a/cpu-exec.c
>> +++ b/cpu-exec.c
>> @@ -352,7 +352,7 @@ static inline TranslationBlock *tb_find_fast(CPUState 
>> *cpu,
>>          /* Check if translation buffer has been flushed */
>>          if (cpu->tb_flushed) {
>>              cpu->tb_flushed = false;
>> -        } else {
>> +        } else if (!tb_is_invalid(tb)) {
>>              tb_add_jump(last_tb, tb_exit, tb);
>>          }
>>      }
>> diff --git a/translate-all.c b/translate-all.c
>> index 788fed1e0765..ee8308209350 100644
>> --- a/translate-all.c
>> +++ b/translate-all.c
>> @@ -986,11 +986,13 @@ static inline void tb_jmp_unlink(TranslationBlock *tb)
>>  /* invalidate one TB */
>>  void tb_phys_invalidate(TranslationBlock *tb, tb_page_addr_t page_addr)
>>  {
>> -    CPUState *cpu;
>>      PageDesc *p;
>>      uint32_t h;
>>      tb_page_addr_t phys_pc;
>>
>> +    tb_mark_invalid(tb);
>> +    smp_wmb();
>> +
>>      /* remove the TB from the hash list */
>>      phys_pc = tb->page_addr[0] + (tb->pc & ~TARGET_PAGE_MASK);
>>      h = tb_hash_func(phys_pc, tb->pc, tb->flags);
>> @@ -1008,14 +1010,6 @@ void tb_phys_invalidate(TranslationBlock *tb, 
>> tb_page_addr_t page_addr)
>>          invalidate_page_bitmap(p);
>>      }
>>
>> -    /* remove the TB from the hash list */
>> -    h = tb_jmp_cache_hash_func(tb->pc);
>> -    CPU_FOREACH(cpu) {
>> -        if (atomic_read(&cpu->tb_jmp_cache[h]) == tb) {
>> -            atomic_set(&cpu->tb_jmp_cache[h], NULL);
>> -        }
>> -    }
>> -
>>      /* suppress this TB from the two jump lists */
>>      tb_remove_from_jmp_list(tb, 0);
>>      tb_remove_from_jmp_list(tb, 1);
>
> --
> Alex Bennée