Re: [Qemu-devel] [RFC PATCH V6 14/18] add a callback when tb_invalidate is called.

[Frederic Konrad <fred.konrad@greensocs.com>]

On 26/06/2015 18:20, Paolo Bonzini wrote:
>
> On 26/06/2015 16:47, fred.konrad@greensocs.com wrote:
>> From: KONRAD Frederic <fred.konrad@greensocs.com>
>>
>> Instead of doing the jump cache invalidation directly in tb_invalidate delay it
>> after the exit so we don't have an other CPU trying to execute the code being
>> invalidated.
>>
>> Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
>> ---
>>   translate-all.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>>   1 file changed, 59 insertions(+), 2 deletions(-)
>>
>> diff --git a/translate-all.c b/translate-all.c
>> index ade2269..468648d 100644
>> --- a/translate-all.c
>> +++ b/translate-all.c
>> @@ -61,6 +61,7 @@
>>   #include "translate-all.h"
>>   #include "qemu/bitmap.h"
>>   #include "qemu/timer.h"
>> +#include "sysemu/cpus.h"
>>   
>>   //#define DEBUG_TB_INVALIDATE
>>   //#define DEBUG_FLUSH
>> @@ -966,14 +967,58 @@ static inline void tb_reset_jump(TranslationBlock *tb, int n)
>>       tb_set_jmp_target(tb, n, (uintptr_t)(tb->tc_ptr + tb->tb_next_offset[n]));
>>   }
>>   
>> +struct CPUDiscardTBParams {
>> +    CPUState *cpu;
>> +    TranslationBlock *tb;
>> +};
>> +
>> +static void cpu_discard_tb_from_jmp_cache(void *opaque)
>> +{
>> +    unsigned int h;
>> +    struct CPUDiscardTBParams *params = opaque;
>> +
>> +    h = tb_jmp_cache_hash_func(params->tb->pc);
>> +    if (params->cpu->tb_jmp_cache[h] == params->tb) {
>> +        params->cpu->tb_jmp_cache[h] = NULL;
>> +    }
> It is a bit more tricky, but I think you can avoid async_run_on_cpu by
> doing this:
>
> 1) introduce a QemuSeqLock in TBContext, e.g. invalidate_seqlock.
>
> 2) wrap this "if" with seqlock_write_lock/unlock
>
> 3) in cpu-exec.c do this:
>
>       /* we add the TB in the virtual pc hash table */
> +    idx = seqlock_read_begin(&tcg_ctx.tb_ctx.invalidate_seqlock);
>       cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)] = tb;
> +    if (seqlock_read_retry(&tcg_ctx.tb_ctx.invalidate_seqlock)) {
> +        /* Another CPU invalidated a tb in the meanwhile.  We do not
> +         * know if it's this one, but play it safe and avoid caching
> +         * it.
> +         */
> +        cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)] = NULL;
> +    }
>
>> +    /* suppress this TB from the two jump lists */
>> +    tb_jmp_remove(tb, 0);
>> +    tb_jmp_remove(tb, 1);
> If you do the above synchronously, this part doesn't need to be deferred
> either.
>
> Then, immediately after the two tb_jmp_remove calls you can also check
> whether "(tb->jmp_first & 3) == 2": if so, the expensive expensive
> async_run_safe_work_on_cpu can be skipped.
>
> Paolo
Ok seems tricky :) I'll take a look at this.

Fred

>> +#endif /* MTTCG */
>>   
>>       tcg_ctx.tb_ctx.tb_phys_invalidate_count++;
>>       tb_unlock();
>>