Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] Add save-snapshot, load-snapshot and delete-snapshot to QAPI

[Dr. David Alan Gilbert]

* Roman Kagan (address@hidden) wrote:
> On Tue, Feb 13, 2018 at 03:05:03PM +0000, Dr. David Alan Gilbert wrote:
> > * Denis V. Lunev (address@hidden) wrote:
> > > On 02/13/2018 05:59 PM, Dr. David Alan Gilbert wrote:
> > > > * Daniel P. Berrangé (address@hidden) wrote:
> > > >> That doesn't seem practical unless you can instantaneously write out
> > > >> the entire guest RAM to disk without blocking, or can somehow snapshot
> > > >> the RAM so you can write out a consistent view of the original RAM,
> > > >> while the guest continues to dirty RAM pages.
> > > > People have suggested doing something like that with userfault write
> > > > mode; but the same would also be doable just by write protecting the
> > > > whole of RAM and then following the faults.
> > > 
> > > nope, userfault fd does not help :( We have tried, the functionality is 
> > > not
> > > enough. Better to have small extension to KVM to protect all memory
> > > and notify QEMU with accessed address.
> > 
> > Can you explain why? I thought the write-protect mode of userfaultfd was
> > supposed to be able to do that; cc'ing in Andrea
> 
> IIRC it would if it worked; it just didn't when we tried.

Hmm that doesn't seem to be the ideal reason to create new KVM
functionality, especially since there were a bunch of people wanting the
userfaultfd-write mode for other uses.

Dave

> Roman.
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK