Re: [Qemu-devel] [RFC PATCH RDMA support v4: 03/10] more verbose documentation of the RDMA transport

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 20, 2013 at 12:08:40PM -0400, Michael R. Hines wrote:
> 
> On 03/20/2013 11:55 AM, Michael S. Tsirkin wrote:
> >On Wed, Mar 20, 2013 at 11:15:48AM -0400, Michael R. Hines wrote:
> >>OK, can we make a deal? =)
> >>
> >>I'm willing to put in the work to perform the dynamic registration
> >>on the destination side,
> >>but let's go a step further and piggy-back on the effort:
> >>
> >>We need to couple this registration with a very small modification
> >>to save_ram_block():
> >>
> >>Currently, save_ram_block does:
> >>
> >>1. is RDMA turned on?      if yes, unconditionally add to next chunk
> >>                                          (will be made to
> >>dynamically register on destination)
> >>2. is_dup_page() ?            if yes, skip
> >>3. in xbzrle cache?           if yes, skip
> >>4. still not sent?                if yes, transmit
> >>
> >>I propose adding a "stub" function that adds:
> >>
> >>0. is page mapped?         if yes, skip   (always returns true for now)
> >>1. same
> >>2. same
> >>3. same
> >>4. same
> >>
> >>Then, later, in a separate patch, I can implement /dev/pagemap support.
> >>
> >>When that's done, RDMA dynamic registration will actually take effect and
> >>benefit from actually verifying that the page is mapped or not.
> >>
> >>- Michael
> >Mapped into guest? You mean e.g. for ballooning?
> >
> 
> No, not just ballooning. Overcommit (i.e. cgroups).
> 
> Anytime cgroups kicks out a page (or anytime the balloon kicks in),
> the page would become unmapped.

OK but we still need to send that page to remote.
It's in swap but has guest data in there, you can't
just ignore it.

> The make dynamic registration useful, we have to actually have something
> in place in the future that knows how to *check* if a page is unmapped
> from the virtual machine, either because it has never been dirtied before
> (and might be pointing to the zero page) or because it has been madvised()
> out or has been detatched because of a cgroup limit.
> 
> - Michael
>