Re: [PATCH v4 11/34] migration/ram: Introduce 'fixed-ram' migration capability

[Peter Xu <peterx@redhat.com>]

On Tue, Feb 20, 2024 at 07:41:15PM -0300, Fabiano Rosas wrote:
> + Without fixed-ram:                  With fixed-ram:
> +
> + ---------------------               --------------------------------
> + | 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:

Super-nit: you can drop the " " otherwise it's put into the quote.

> +  - ramblock header: the generic information for a ramblock, such as
> +    idstr, used_len, etc.
> +
> +  - ramblock fixed-ram header: the information added by this feature:
> +    bitmap of pages written, bitmap size and offset of pages in the
> +    migration file.
> +
> +Restrictions
> +------------
> +
> +Since pages are written to their relative offsets and out of order
> +(due to the memory dirtying patterns), streaming channels such as
> +sockets are not supported. A seekable channel such as a file is
> +required. This can be verified in the QIOChannel by the presence of
> +the QIO_CHANNEL_FEATURE_SEEKABLE.

Would it worth also mention that it only provides fixed offsets to "guest
physical RAM" only?  For example, GPU RAM won't apply as they're migrated
as part of device states, even if also iterable.  IOW, IIUC if there's a
VFIO device (or a few), the fixed-ram migration file will still be unbound
in size, because device can keep flushing stale vRAM to the image..

Maybe that's too specific, I'll leave that to you to decide whether to even
mention it.

-- 
Peter Xu