Re: Reducing vdpa migration downtime because of memory pin / maps

[Longpeng (Mike, Cloud Infrastructure Service Product Dept.)]

在 2023/4/10 10:14, Jason Wang 写道:
> On Wed, Apr 5, 2023 at 7:38 PM Eugenio Perez Martin <eperezma@redhat.com> wrote:
> > Hi!
> >
> > As mentioned in the last upstream virtio-networking meeting, one of
> > the factors that adds more downtime to migration is the handling of
> > the guest memory (pin, map, etc). At this moment this handling is
> > bound to the virtio life cycle (DRIVER_OK, RESET). In that sense, the
> > destination device waits until all the guest memory / state is
> > migrated to start pinning all the memory.
> >
> > The proposal is to bind it to the char device life cycle (open vs
> > close), so all the guest memory can be pinned for all the guest / qemu
> > lifecycle.
> >
> > This has two main problems:
> > * At this moment the reset semantics forces the vdpa device to unmap
> > all the memory. So this change needs a vhost vdpa feature flag.
>
> Is this true? I didn't find any codes to unmap the memory in
> vhost_vdpa_set_status().

It could depend on the vendor driver, for example, the vdpasim would do 
something like that.

vhost_vdpa_set_status->vdpa_reset->vdpasim_reset->vdpasim_do_reset->vhost_iotlb_reset

> Thanks
>
> > * This may increase the initialization time. Maybe we can delay it if
> > qemu is not the destination of a LM. Anyway I think this should be
> > done as an optimization on top.
> >
> > Any ideas or comments in this regard?
> >
> > Thanks!
>
> .