Re: [Qemu-devel] [PATCH v3 03/11] tcg: Prepare safe tb_jmp_cache lookup out of tb_lock

[Alex Bennée]

Sergey Fedorov <address@hidden> writes:

> From: Sergey Fedorov <address@hidden>
>
> Ensure atomicity of CPU's 'tb_jmp_cache' access for future translation
> block lookup out of 'tb_lock'.
>
> Note that this patch does *not* make CPU's TLB invalidation safe if it
> is done from some other thread while the CPU is in its execution loop.
>
> Signed-off-by: Sergey Fedorov <address@hidden>
> Signed-off-by: Sergey Fedorov <address@hidden>
> [AJB: fixed missing atomic set, tweak title]
> Signed-off-by: Alex Bennée <address@hidden>
> [Sergey Fedorov: removed explicit memory barriers;
> memset() on 'tb_jmp_cache' replaced with a loop on atomic_set();
> tweaked commit title and message]
> Signed-off-by: Sergey Fedorov <address@hidden>

Reviewed-by: Alex Bennée <address@hidden>

Not sure it counts as it's already had my s-o-b when I last had the tree
but there you go ;-)

>
> ---
> Changes in v3:
>  - explicit memory barriers removed
>  - memset() on 'tb_jmp_cache' replaced with a loop on atomic_set()
> Changes in v2:
>  - fix spelling s/con't/can't/
>  - add atomic_read while clearing tb_jmp_cache
>  - add r-b tags
> Changes in v1 (AJB):
>  - tweak title
>  - fixed missing set of tb_jmp_cache
> ---
>  cpu-exec.c      |  4 ++--
>  translate-all.c | 10 +++++++---
>  2 files changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/cpu-exec.c b/cpu-exec.c
> index 757f63b08b6b..d6178eab71d4 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -315,7 +315,7 @@ static TranslationBlock *tb_find_slow(CPUState *cpu,
>
>  found:
>      /* we add the TB in the virtual pc hash table */
> -    cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)] = tb;
> +    atomic_set(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)], tb);
>      return tb;
>  }
>
> @@ -333,7 +333,7 @@ static inline TranslationBlock *tb_find_fast(CPUState 
> *cpu,
>         is executed. */
>      cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
>      tb_lock();
> -    tb = cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)];
> +    tb = atomic_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]);
>      if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
>                   tb->flags != flags)) {
>          tb = tb_find_slow(cpu, pc, cs_base, flags);
> diff --git a/translate-all.c b/translate-all.c
> index 0d47c1c0cf82..fdf520a86d68 100644
> --- a/translate-all.c
> +++ b/translate-all.c
> @@ -848,7 +848,11 @@ void tb_flush(CPUState *cpu)
>      tcg_ctx.tb_ctx.nb_tbs = 0;
>
>      CPU_FOREACH(cpu) {
> -        memset(cpu->tb_jmp_cache, 0, sizeof(cpu->tb_jmp_cache));
> +        int i;
> +
> +        for (i = 0; i < TB_JMP_CACHE_SIZE; ++i) {
> +            atomic_set(&cpu->tb_jmp_cache[i], NULL);
> +        }
>          cpu->tb_flushed = true;
>      }
>
> @@ -1007,8 +1011,8 @@ void tb_phys_invalidate(TranslationBlock *tb, 
> tb_page_addr_t page_addr)
>      /* remove the TB from the hash list */
>      h = tb_jmp_cache_hash_func(tb->pc);
>      CPU_FOREACH(cpu) {
> -        if (cpu->tb_jmp_cache[h] == tb) {
> -            cpu->tb_jmp_cache[h] = NULL;
> +        if (atomic_read(&cpu->tb_jmp_cache[h]) == tb) {
> +            atomic_set(&cpu->tb_jmp_cache[h], NULL);
>          }
>      }


--
Alex Bennée