Re: [Qemu-devel] [RFC v2] translate-all: protect code_gen_buffer with RCU

["Alex Bennée" <alex.bennee@linaro.org>]

Emilio G. Cota <cota@braap.org> writes:

> [ Applies on top of bennee/mttcg/enable-mttcg-for-armv7-v1 after
> reverting "translate-all: introduces tb_flush_safe". A trivial
> conflict must be solved after applying. ]
>
> This is a first attempt at making tb_flush not have to stop all CPUs.
> There are issues as pointed out below, but this could be a good start.
>
> Context:
>   https://lists.gnu.org/archive/html/qemu-devel/2016-03/msg04658.html
>   https://lists.gnu.org/archive/html/qemu-devel/2016-03/msg06942.html
>
> Changes from v1:
> - When a static buffer is used, split it in two instead of using
>   a second buffer.
>
> Known issues:
> - Fails Alex' unit test with low enough -tb-size, see
>   https://lists.gnu.org/archive/html/qemu-devel/2016-04/msg03465.html
>   Seems to work in MTTCG, although I've only tested with tb_lock
>   always being taken in tb_find_fast.

With --enable-debug-tcg I get it failing pretty quickly:

#4  0x00005555556d332a in tcg_global_alloc (s=0x555556007ba0 <tcg_ctx>)
 at /home/alex/lsrc/qemu/qemu.git/tcg/tcg.c:463
 463         tcg_debug_assert(s->nb_globals == s->nb_temps);
 (gdb) p s->nb_globals
 $1 = 24
 (gdb) p s->nb_temps
 $2 = 31

Seems odd though, the other threads are all waiting on the tb_lock.

> - Windows; not even compile-tested!
>
> Signed-off-by: Emilio G. Cota <cota@braap.org>
> ---
>  translate-all.c | 146 +++++++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 133 insertions(+), 13 deletions(-)
>
> diff --git a/translate-all.c b/translate-all.c
> index 8e70583..6830371 100644
> --- a/translate-all.c
> +++ b/translate-all.c
> @@ -535,6 +535,9 @@ static inline void *split_cross_256mb(void *buf1, size_t size1)
>  #ifdef USE_STATIC_CODE_GEN_BUFFER
>  static uint8_t static_code_gen_buffer[DEFAULT_CODE_GEN_BUFFER_SIZE]
>      __attribute__((aligned(CODE_GEN_ALIGN)));
> +static int static_buf_mask = 1;
> +static void *static_buf1;
> +static void *static_buf2;
>
>  # ifdef _WIN32
>  static inline void do_protect(void *addr, long size, int prot)
> @@ -577,6 +580,13 @@ static inline void map_none(void *addr, long size)
>  }
>  # endif /* WIN32 */
>
> +static void map_static_code_gen_buffer(void *buf, size_t size)
> +{
> +    map_exec(buf, size);
> +    map_none(buf + size, qemu_real_host_page_size);
> +    qemu_madvise(buf, size, QEMU_MADV_HUGEPAGE);
> +}
> +
>  static inline void *alloc_code_gen_buffer(void)
>  {
>      void *buf = static_code_gen_buffer;
> @@ -586,28 +596,41 @@ static inline void *alloc_code_gen_buffer(void)
>      full_size = (((uintptr_t)buf + sizeof(static_code_gen_buffer))
>                   & qemu_real_host_page_mask) - (uintptr_t)buf;
>
> -    /* Reserve a guard page.  */
> -    size = full_size - qemu_real_host_page_size;
> +    /*
> +     * Reserve two guard pages, one after each of the two buffers:
> +     * |     buf1     |g1|    buf2    |g2|
> +     */
> +    size = full_size - 2 * qemu_real_host_page_size;
>
>      /* Honor a command-line option limiting the size of the buffer.  */
>      if (size > tcg_ctx.code_gen_buffer_size) {
>          size = (((uintptr_t)buf + tcg_ctx.code_gen_buffer_size)
>                  & qemu_real_host_page_mask) - (uintptr_t)buf;
>      }
> -    tcg_ctx.code_gen_buffer_size = size;
>
>  #ifdef __mips__
> -    if (cross_256mb(buf, size)) {
> -        buf = split_cross_256mb(buf, size);
> -        size = tcg_ctx.code_gen_buffer_size;
> +    /*
> +     * Pass 'size + page_size', since we want 'buf1 | guard1 | buf2' to be
> +     * within the boundary.
> +     */
> +    if (cross_256mb(buf, size + qemu_real_host_page_size)) {
> +        buf = split_cross_256mb(buf, size + qemu_real_host_page_size);
> +        size = tcg_ctx.code_gen_buffer_size - qemu_real_host_page_size;
>      }
>  #endif
>
> -    map_exec(buf, size);
> -    map_none(buf + size, qemu_real_host_page_size);
> -    qemu_madvise(buf, size, QEMU_MADV_HUGEPAGE);
> +    /* split the buffer in two */
> +    size /= 2;
> +    tcg_ctx.code_gen_buffer_size = size;
>
> -    return buf;
> +    static_buf1 = buf;
> +    static_buf2 = buf + size + qemu_real_host_page_mask;
> +
> +    map_static_code_gen_buffer(static_buf1, size);
> +    map_static_code_gen_buffer(static_buf2, size);
> +
> +    assert(static_buf_mask == 1);
> +    return static_buf1;
>  }
>  #elif defined(_WIN32)
>  static inline void *alloc_code_gen_buffer(void)
> @@ -825,10 +848,100 @@ static void page_flush_tb(void)
>      }
>  }
>
> +#ifdef USE_STATIC_CODE_GEN_BUFFER
> +
> +struct code_gen_desc {
> +    struct rcu_head rcu;
> +    int clear_bit;
> +};
> +
> +static void code_gen_buffer_clear(struct rcu_head *rcu)
> +{
> +    struct code_gen_desc *desc = container_of(rcu, struct code_gen_desc, rcu);
> +
> +    tb_lock();
> +    static_buf_mask &= ~desc->clear_bit;
> +    tb_unlock();
> +    g_free(desc);
> +}
> +
> +static void *code_gen_buffer_replace(void)
> +{
> +    struct code_gen_desc *desc = g_malloc0(sizeof(*desc));
> +
> +    /*
> +     * If both bits are set, we're having two concurrent flushes. This
> +     * can easily happen if the buffers are heavily undersized.
> +     */
> +    assert(static_buf_mask == 1 || static_buf_mask == 2);
> +
> +    desc->clear_bit = static_buf_mask;
> +    call_rcu1(&desc->rcu, code_gen_buffer_clear);
> +
> +    if (static_buf_mask == 1) {
> +        static_buf_mask |= 2;
> +        return static_buf2;
> +    }
> +    static_buf_mask |= 1;
> +    return static_buf1;
> +}
> +
> +#elif defined(_WIN32)
> +
> +struct code_gen_desc {
> +    struct rcu_head rcu;
> +    void *buf;
> +};
> +
> +static void code_gen_buffer_vfree(struct rcu_head *rcu)
> +{
> +    struct code_gen_desc *desc = container_of(rcu, struct code_gen_desc, rcu);
> +
> +    VirtualFree(desc->buf, 0, MEM_RELEASE);
> +    g_free(desc);
> +}
> +
> +static void *code_gen_buffer_replace(void)
> +{
> +    struct code_gen_desc *desc;
> +
> +    desc = g_malloc0(sizeof(*desc));
> +    desc->buf = tcg_ctx.code_gen_buffer;
> +    call_rcu1(&desc->rcu, code_gen_buffer_vfree);
> +
> +    return alloc_code_gen_buffer();
> +}
> +
> +#else /* UNIX, dynamically-allocated code buffer */
> +
> +struct code_gen_desc {
> +    struct rcu_head rcu;
> +    void *buf;
> +    size_t size;
> +};
> +
> +static void code_gen_buffer_unmap(struct rcu_head *rcu)
> +{
> +    struct code_gen_desc *desc = container_of(rcu, struct code_gen_desc, rcu);
> +
> +    munmap(desc->buf, desc->size + qemu_real_host_page_size);
> +    g_free(desc);
> +}
> +
> +static void *code_gen_buffer_replace(void)
> +{
> +    struct code_gen_desc *desc;
> +
> +    desc = g_malloc0(sizeof(*desc));
> +    desc->buf = tcg_ctx.code_gen_buffer;
> +    desc->size = tcg_ctx.code_gen_buffer_size;
> +    call_rcu1(&desc->rcu, code_gen_buffer_unmap);
> +
> +    return alloc_code_gen_buffer();
> +}
> +#endif /* USE_STATIC_CODE_GEN_BUFFER */
> +
>  /* flush all the translation blocks */
> -/* XXX: tb_flush is currently not thread safe.  System emulation calls it only
> - * with tb_lock taken or from safe_work, so no need to take tb_lock here.
> - */
>  void tb_flush(CPUState *cpu)
>  {
>  #if defined(DEBUG_FLUSH)
> @@ -852,10 +965,17 @@ void tb_flush(CPUState *cpu)
>      memset(tcg_ctx.tb_ctx.tb_phys_hash, 0, sizeof(tcg_ctx.tb_ctx.tb_phys_hash));
>      page_flush_tb();
>
> +    tcg_ctx.code_gen_buffer = code_gen_buffer_replace();
>      tcg_ctx.code_gen_ptr = tcg_ctx.code_gen_buffer;
> +    tcg_prologue_init(&tcg_ctx);
>      /* XXX: flush processor icache at this point if cache flush is
>         expensive */
>      tcg_ctx.tb_ctx.tb_flush_count++;
> +
> +    /* exit all CPUs so that the old buffer is quickly cleared. */
> +    CPU_FOREACH(cpu) {
> +        cpu_exit(cpu);
> +    }
>  }
>
>  #ifdef DEBUG_TB_CHECK


--
Alex Bennée