Re: [PATCH v4 13/34] migration/ram: Add outgoing 'fixed-ram' migration

[Peter Xu <peterx@redhat.com>]

On Tue, Feb 20, 2024 at 07:41:17PM -0300, Fabiano Rosas wrote:
> Implement the outgoing migration side for the 'fixed-ram' capability.
> 
> A bitmap is introduced to track which pages have been written in the
> migration file. Pages are written at a fixed location for every
> ramblock. Zero pages are ignored as they'd be zero in the destination
> migration as well.
> 
> The migration stream is altered to put the dirty pages for a ramblock
> after its header instead of having a sequential stream of pages that
> follow the ramblock headers.
> 
> Without fixed-ram (current):        With fixed-ram (new):
> 
>  ---------------------               --------------------------------
>  | ramblock 1 header |               | ramblock 1 header            |
>  ---------------------               --------------------------------
>  | ramblock 2 header |               | ramblock 1 fixed-ram header  |
>  ---------------------               --------------------------------
>  | ...               |               | padding to next 1MB boundary |
>  ---------------------               | ...                          |
>  | ramblock n header |               --------------------------------
>  ---------------------               | ramblock 1 pages             |
>  | RAM_SAVE_FLAG_EOS |               | ...                          |
>  ---------------------               --------------------------------
>  | stream of pages   |               | ramblock 2 header            |
>  | (iter 1)          |               --------------------------------
>  | ...               |               | ramblock 2 fixed-ram header  |
>  ---------------------               --------------------------------
>  | RAM_SAVE_FLAG_EOS |               | padding to next 1MB boundary |
>  ---------------------               | ...                          |
>  | stream of pages   |               --------------------------------
>  | (iter 2)          |               | ramblock 2 pages             |
>  | ...               |               | ...                          |
>  ---------------------               --------------------------------
>  | ...               |               | ...                          |
>  ---------------------               --------------------------------
>                                      | RAM_SAVE_FLAG_EOS            |
>                                      --------------------------------
>                                      | ...                          |
>                                      --------------------------------
> 
> where:
>  - ramblock header: the generic information for a ramblock, such as
>    idstr, used_len, etc.
> 
>  - ramblock fixed-ram header: the new information added by this
>    feature: bitmap of pages written, bitmap size and offset of pages
>    in the migration file.
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> Signed-off-by: Fabiano Rosas <farosas@suse.de>

Reviewed-by: Peter Xu <peterx@redhat.com>

Still one comment below:

[...]

> @@ -3187,6 +3288,18 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
>          return ret;
>      }
>  
> +    if (migrate_fixed_ram()) {
> +        ram_save_file_bmap(f);
> +
> +        if (qemu_file_get_error(f)) {
> +            Error *local_err = NULL;
> +            int err = qemu_file_get_error_obj(f, &local_err);
> +
> +            error_reportf_err(local_err, "Failed to write bitmap to file: ");

We always do error report if we set s->error.

Ideally I think we should have Error** passed to the caller and set
s->error there, instead of report here.  But the whole error handling is
still a bit of a mess, so I guess we can do anything on top.

> +            return -err;
> +        }
> +    }

-- 
Peter Xu