On Tue, Apr 25, 2023 at 01:56:03PM +0300, Vladimir Sementsov-Ogievskiy wrote:
On incoming migration we have the following sequence to load option
ROM:
1. On device realize we do normal load ROM from the file
2. Than, on incoming migration we rewrite ROM from the incoming RAM
block. If sizes mismatch we fail.
This is not ideal when we migrate to updated distribution: we have to
keep old ROM files in new distribution and be careful around romfile
property to load correct ROM file. Which is loaded actually just to
allocate the ROM with correct length.
Note, that romsize property doesn't really help: if we try to specify
it when default romfile is larger, it fails with something like:
romfile "efi-virtio.rom" (160768 bytes) is too large for ROM size 65536
This commit brings new behavior for romfile="",romsize=SIZE combination
of options. Current behavior is just ignore romsize and not load or
create any ROM.
Let's instead preallocate ROM, not loading any file. This way we can
migrate old vm to new environment not thinking about ROM files on
destination host:
1. specify romfile="",romsize=SIZE on target, with correct SIZE
(actually, size of romfile on source aligned up to power of two, or
just original romsize option on source)
2. On device realize we just preallocate ROM, and not load any file
3. On incoming migration ROM is filled from the migration stream
As a bonus we avoid extra reading from ROM file on target.
Signed-off-by: Vladimir Sementsov-Ogievskiy<vsementsov@yandex-team.ru>
why is this a bad idea:
- on source presumably user overrides romfile
- we have a general rule that source and destination flags must match
I propose instead to ignore romfile if qemu is incoming migration
and romsize has been specified.