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

[Jason Wang]

On Tue, Apr 11, 2023 at 8:34 PM Eugenio Perez Martin
<eperezma@redhat.com> wrote:
>
> On Wed, Apr 5, 2023 at 1:37 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.
> > * 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.
> >
>
> Expanding on this we could reduce the pinning even more now that vring
> supports VA [1] with the emulated CVQ.

Note that VA for hardware means the device needs to support page fault
through either PRI or vendor specific interface.

>
> Something like:
> - Add VHOST_VRING_GROUP_CAN_USE_VA ioctl to check if a given VQ group
> capability. Passthrough devices with emulated CVQ would return false
> for the dataplane and true for the control vq group.
> - If that is true, qemu does not need to map and translate addresses
> for CVQ but to directly provide VA for buffers. This avoids pinning,
> translations, etc in this case.

For CVQ yes, but we only avoid the pinning for CVQ not others.

Thanks

>
> Thanks!
>
> [1] 
> https://lore.kernel.org/virtualization/20230404131326.44403-2-sgarzare@redhat.com/
>