Re: [Qemu-devel] [RFC 26/29] vhost: Add VHOST_USER_POSTCOPY_END message

[Dr. David Alan Gilbert]

* Maxime Coquelin (address@hidden) wrote:
> 
> 
> On 06/28/2017 09:00 PM, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> > 
> > This message is sent just before the end of postcopy to get the
> > client to stop using userfault since we wont respond to any more
> > requests.  It should close userfaultfd so that any other pages
> > get mapped to the backing file automatically by the kernel, since
> > at this point we know we've received everything.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > ---
> >   contrib/libvhost-user/libvhost-user.c | 23 +++++++++++++++++++++++
> >   contrib/libvhost-user/libvhost-user.h |  1 +
> >   hw/virtio/vhost-user.c                |  1 +
> >   3 files changed, 25 insertions(+)
> > 
> > diff --git a/contrib/libvhost-user/libvhost-user.c 
> > b/contrib/libvhost-user/libvhost-user.c
> > index d37052b7b0..c1716d1a62 100644
> > --- a/contrib/libvhost-user/libvhost-user.c
> > +++ b/contrib/libvhost-user/libvhost-user.c
> > @@ -68,6 +68,7 @@ vu_request_to_string(int req)
> >           REQ(VHOST_USER_INPUT_GET_CONFIG),
> >           REQ(VHOST_USER_POSTCOPY_ADVISE),
> >           REQ(VHOST_USER_POSTCOPY_LISTEN),
> > +        REQ(VHOST_USER_POSTCOPY_END),
> >           REQ(VHOST_USER_MAX),
> >       };
> >   #undef REQ
> > @@ -889,6 +890,26 @@ vu_set_postcopy_listen(VuDev *dev, VhostUserMsg *vmsg)
> >       return false;
> >   }
> > +
> > +static bool
> > +vu_set_postcopy_end(VuDev *dev, VhostUserMsg *vmsg)
> > +{
> > +    fprintf(stderr, "%s: Entry\n", __func__);
> > +    dev->postcopy_listening = false;
> > +    if (dev->postcopy_ufd > 0) {
> > +        close(dev->postcopy_ufd);
> > +        dev->postcopy_ufd = -1;
> > +        fprintf(stderr, "%s: Done close\n", __func__);
> > +    }
> > +
> > +    vmsg->fd_num = 0;
> > +    vmsg->payload.u64 = 0;
> > +    vmsg->size = sizeof(vmsg->payload.u64);
> > +    vmsg->flags = VHOST_USER_VERSION |  VHOST_USER_REPLY_MASK;
> > +    fprintf(stderr, "%s: exit\n", __func__);
> > +    return true;
> > +}
> > +
> 
> It is what reply-ack is done for, so to avoid code duplication,
> maybe Qemu could set VHOST_USER_NEED_REPLY_MASK bit when reply-ack
> feature is supported.
> 
> I'm wondering if we shouldn't consider adding reply-ack feature support
> to libvhost-user, and make postcopy support to depend on this feature.

Yes, that would make sense;  for the moment I'm adding what I think is
the same replies in the places I need ack's, and setting the 
VHOST_USER_NEED_REPLY_MASK
so for the messages where I need it the semantics should be the same as
that.

Dave

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