Re: [PATCH v7 12/12] So we use multifd to transmit zero pages.

[Leonardo Brás]

On Tue, 2022-08-02 at 08:39 +0200, Juan Quintela wrote:
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> 
> ---
> 
> - Check zero_page property before using new code (Dave)
> ---
>  migration/migration.c |  4 +---
>  migration/multifd.c   |  6 +++---
>  migration/ram.c       | 33 ++++++++++++++++++++++++++++++++-
>  3 files changed, 36 insertions(+), 7 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index ce3e5cc0cd..13842f6803 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -2599,9 +2599,7 @@ bool migrate_use_main_zero_page(void)
>  
>      s = migrate_get_current();
>  
> -    // We will enable this when we add the right code.
> -    // return s->enabled_capabilities[MIGRATION_CAPABILITY_MAIN_ZERO_PAGE];
> -    return true;
> +    return s->enabled_capabilities[MIGRATION_CAPABILITY_MAIN_ZERO_PAGE];
>  }
>  
>  bool migrate_pause_before_switchover(void)
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 89811619d8..54acdc004c 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -667,8 +667,8 @@ static void *multifd_send_thread(void *opaque)
>  {
>      MultiFDSendParams *p = opaque;
>      Error *local_err = NULL;
> -    /* qemu older than 7.0 don't understand zero page on multifd channel */
> -    bool use_zero_page = migrate_use_multifd_zero_page();
> +    /* older qemu don't understand zero page on multifd channel */
> +    bool use_multifd_zero_page = !migrate_use_main_zero_page();

I understand that "use_main_zero_page", which is introduced as a new capability,
is in fact the old behavior, and the new feature is introduced when this
capability is disabled.

But it sure looks weird reading:
 use_multifd_zero_page = !migrate_use_main_zero_page();

This series is fresh in my mind, but it took a few seconds to see that this is
actually not a typo. 

>      int ret = 0;
>      bool use_zero_copy_send = migrate_use_zero_copy_send();
>  
> @@ -711,7 +711,7 @@ static void *multifd_send_thread(void *opaque)
>  
>              for (int i = 0; i < p->pages->num; i++) {
>                  uint64_t offset = p->pages->offset[i];
> -                if (use_zero_page &&
> +                if (use_multifd_zero_page &&
>                      buffer_is_zero(rb->host + offset, p->page_size)) {
>                      p->zero[p->zero_num] = offset;
>                      p->zero_num++;
> diff --git a/migration/ram.c b/migration/ram.c
> index 2af70f517a..26e60b9cc1 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -2428,6 +2428,32 @@ static void postcopy_preempt_reset_channel(RAMState 
> *rs)
>      }
>  }
>  
> +/**
> + * ram_save_target_page_multifd: save one target page
> + *
> + * Returns the number of pages written
> + *
> + * @rs: current RAM state
> + * @pss: data about the page we want to send
> + */
> +static int ram_save_target_page_multifd(RAMState *rs, PageSearchStatus *pss)
> +{
> +    RAMBlock *block = pss->block;
> +    ram_addr_t offset = ((ram_addr_t)pss->page) << TARGET_PAGE_BITS;
> +    int res;
> +
> +    if (!migration_in_postcopy()) {
> +        return ram_save_multifd_page(rs, block, offset);
> +    }
> +
> +    res = save_zero_page(rs, block, offset);
> +    if (res > 0) {
> +        return res;
> +    }
> +
> +    return ram_save_page(rs, pss);
> +}
> +
>  /**
>   * ram_save_host_page: save a whole host page
>   *
> @@ -3225,7 +3251,12 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>      ram_control_before_iterate(f, RAM_CONTROL_SETUP);
>      ram_control_after_iterate(f, RAM_CONTROL_SETUP);
>  
> -    (*rsp)->ram_save_target_page = ram_save_target_page_legacy;
> +    if (migrate_use_multifd() && !migrate_use_main_zero_page()) {
> +        (*rsp)->ram_save_target_page = ram_save_target_page_multifd;
> +    } else {
> +        (*rsp)->ram_save_target_page = ram_save_target_page_legacy;
> +    }
> +
>      ret =  multifd_send_sync_main(f);
>      if (ret < 0) {
>          return ret;

The rest LGTM.

FWIW:
Reviewed-by: Leonardo Bras <leobras@redhat.com>