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

[Paolo Bonzini <pbonzini@redhat.com>]

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

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