Re: [PATCH 5/6] migration/postcopy: enable random order target page arrival

[Dr. David Alan Gilbert]

* Wei Yang (address@hidden) wrote:
> After using number of target page received to track one host page, we
> could have the capability to handle random order target page arrival in
> one host page.
> 
> This is a preparation for enabling compress during postcopy.
> 
> Signed-off-by: Wei Yang <address@hidden>
> ---
>  migration/ram.c | 16 +++-------------
>  1 file changed, 3 insertions(+), 13 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index b5759793a9..da0596411c 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -4015,7 +4015,6 @@ static int ram_load_postcopy(QEMUFile *f)
>      MigrationIncomingState *mis = migration_incoming_get_current();
>      /* Temporary page that is later 'placed' */
>      void *postcopy_host_page = mis->postcopy_tmp_page;
> -    void *last_host = NULL;
>      bool all_zero = false;
>      int target_pages = 0;
>  
> @@ -4062,24 +4061,15 @@ static int ram_load_postcopy(QEMUFile *f)
>               * that's moved into place later.
>               * The migration protocol uses,  possibly smaller, target-pages
>               * however the source ensures it always sends all the components
> -             * of a host page in order.
> +             * of a host page in one chunk.
>               */
>              page_buffer = postcopy_host_page +
>                            ((uintptr_t)host & (block->page_size - 1));
>              /* If all TP are zero then we can optimise the place */
>              if (target_pages == 1) {
>                  all_zero = true;
> -            } else {
> -                /* not the 1st TP within the HP */
> -                if (host != (last_host + TARGET_PAGE_SIZE)) {
> -                    error_report("Non-sequential target page %p/%p",
> -                                  host, last_host);
> -                    ret = -EINVAL;
> -                    break;
> -                }

I think this is losing more protection than needed.
I think you can still protect against a page from a different host-page
arriving until we've placed the current host-page.
So something like:

    if (((uintptr_t)host & ~(block->page_size - 1)) !=
        last_host)

and then set last_host to the start of the host page.

Then you'll check if that flush is really working.

Dave

>              }
>  
> -
>              /*
>               * If it's the last part of a host page then we place the host
>               * page
> @@ -4090,7 +4080,6 @@ static int ram_load_postcopy(QEMUFile *f)
>              }
>              place_source = postcopy_host_page;
>          }
> -        last_host = host;
>  
>          switch (flags & ~RAM_SAVE_FLAG_CONTINUE) {
>          case RAM_SAVE_FLAG_ZERO:
> @@ -4143,7 +4132,8 @@ static int ram_load_postcopy(QEMUFile *f)
>  
>          if (!ret && place_needed) {
>              /* This gets called at the last target page in the host page */
> -            void *place_dest = host + TARGET_PAGE_SIZE - block->page_size;
> +            void *place_dest = (void *)QEMU_ALIGN_DOWN((unsigned long)host,
> +                                                       block->page_size);
>  
>              if (all_zero) {
>                  ret = postcopy_place_page_zero(mis, place_dest,
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK