Re: [PATCH v3 3/3] vdpa: commit all host notifier MRs in a single MR transaction

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

On Tue, Dec 27, 2022 at 05:51:47PM +0100, Philippe Mathieu-Daudé wrote:
> On 27/12/22 08:20, Longpeng(Mike) wrote:
> > From: Longpeng <longpeng2@huawei.com>
> > 
> > This allows the vhost-vdpa device to batch the setup of all its MRs of
> > host notifiers.
> > 
> > This significantly reduces the device starting time, e.g. the time spend
> > on setup the host notifier MRs reduce from 423ms to 32ms for a VM with
> > 64 vCPUs and 3 vhost-vDPA generic devices (vdpa_sim_blk, 64vq per device).
> > 
> > Signed-off-by: Longpeng <longpeng2@huawei.com>
> > ---
> >   hw/virtio/vhost-vdpa.c | 25 +++++++++++++++++++------
> >   1 file changed, 19 insertions(+), 6 deletions(-)
> > 
> > diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> > index fd0c33b0e1..870265188a 100644
> > --- a/hw/virtio/vhost-vdpa.c
> > +++ b/hw/virtio/vhost-vdpa.c
> > @@ -512,9 +512,18 @@ static void vhost_vdpa_host_notifiers_uninit(struct vhost_dev *dev, int n)
> >   {
> >       int i;
> > +    /*
> > +     * Pack all the changes to the memory regions in a single
> > +     * transaction to avoid a few updating of the address space
> > +     * topology.
> > +     */
> > +    memory_region_transaction_begin();
> > +
> >       for (i = dev->vq_index; i < dev->vq_index + n; i++) {
> >           vhost_vdpa_host_notifier_uninit(dev, i);
> >       }
> > +
> > +    memory_region_transaction_commit();
> >   }
> 
> Instead of optimizing one frontend, I wonder if we shouldn't expose
> a 'bool memory_region_transaction_in_progress()' helper in memory.c,
> and in virtio_queue_set_host_notifier_mr() backend, assert we are
> within a transaction. That way we'd optimize all virtio frontends.


If we are doing something like this, I'd rather pass around
a "transaction" structure so this can be checked statically.
Looks like something that can be done on top though.

-- 
MST