Re: [Qemu-devel] RFC: moving fsfreeze support from the userland guest agent to the guest kernel

[Christoph Hellwig <hch@lst.de>]

Initiating the freeze from kernelspace doesn't make much sense.  With
virtio we could add in-band freeze request to the protocol, and although
that would be a major change in that way virtio-blk works right now it's
at least doable.  But all other "real" storage targets only communicate
with their initators over out of band procotols that are entirely handled
in userspace, and given their high-level nature better are - that is if
we know them at all given how vendors like to keep this secrete IP
closed and just offer userspace management tools in binary form.

building new infrastructure in the kernel just for virtio, while needing
to duplicate the same thing in userspace for all real storage seems like
a really bad idea.  That is in addition to the userspace freeze notifier
similar to what e.g. Windows has - if the freeze process is driven from
userspace it's much easier to handle those properly compared to requiring
kernel upcalls.