Re: [PATCH] migration: add FEATURE_SEEKABLE to QIOChannelBlock

["Marco Cavenati" <Marco.Cavenati@eurecom.fr>]

On Thursday, April 10, 2025 21:52 CEST, Fabiano Rosas <farosas@suse.de> wrote:

> We'll need to add the infrastructure to reject multifd and direct-io
> before this. The rest of the capabilities should not affect mapped-ram,
> so it's fine (for now) if we don't honor them.

Ok, thanks for the update.
 
> What about zero page handling? Mapped-ram doesn't send zero pages
> because the file will always have zeroes in it and the migration
> destination is guaranteed to not have been running previously. I believe
> loading a snapshot in a VM that's already been running would leave stale
> data in the guest's memory.

Yes, you are correct.

About the `RAMBlock->file_bmap`, according to the code it is a:
`/* bitmap of pages present in the migration file */`
And, if a pages is a zero page, it won't be in the migration file:
`/* zero pages are not transferred with mapped-ram */`
So, zero page implies bitmap 0.
Does the opposite hold?

If bitmap 0 implies zero page, we could call `ram_handle_zero`
in `read_ramblock_mapped_ram` for the clear bits.
Or do you fear this might be unnecessary expensive for migration?

If bitmap 0 does not imply zero page, I feel like the
"is present in the migration file" and "is zero page" info should
be better separated.

Best,
Marco