Re: [Qemu-devel] [PATCH v2 3/6] vhost: extend ring information update for IOTLB to all rings

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

On Tue, May 30, 2017 at 11:06:54PM +0200, Maxime Coquelin wrote:
> Hi Michael,
> 
> On 05/30/2017 08:12 PM, Michael S. Tsirkin wrote:
> > On Fri, May 26, 2017 at 04:28:55PM +0200, Maxime Coquelin wrote:
> > > Vhost-kernel backend need
> > 
> > needs
> > 
> > > to receive IOTLB entry for used ring
> > > information early, which is done by triggering a miss event on
> > > its address.
> > > 
> > > This patch extends this behaviour to all rings information, to be
> > > compatible with vhost-user backend design.
> > 
> > Why does vhost-user need it though?
> 
> For vhost-user, this simplifies the backend design because generally,
> the backend sends IOTLB miss requests from processing threads through
> the slave channel, and receives resulting IOTLB updates in vhost-user
> protocol thread.
> 
> The only exception is for these rings info, where IOTLB miss requests
> are sent from vhost-user protocol thread (in the SET_VRING_ENABLE
> request handler), so the resulting IOTLB update is only handled by
> the backend when the rings are enabled, which is too late.
> 
> It could be possible to overcome this issue, but I think it would
> make the design more complex or less efficient. I see several options:
> 
> 1. Change the IOTLB miss request so that the master sends the IOTLB
> update as reply, instead of the reply-ack. It would mean that IOTLB
> updates/invalidations would be sent either via the master channel or
> the slave channel. On QEMU side, it means diverging from kernel backend
> implementation. On backend side, it means having possibly multiple
> threads writing to the IOTLB cache.
> 
> 2. In vhost-user protocol thread, when handling SET_VRING_ENABLE, send
> IOTLB miss request without setting the reply-ack flag, and poll the
> vhost-user socket to read the resulting IOTLB update. The problem is
> that other requests could be pending in the socket's buffer, and so it
> would end-up nesting multiple requests handling.
> 
> 3. Don't interpret rings info in the vhost-user protocol thread, but
> only in the processing threads. The advantage is that it would address
> the remark you made on patch 6 that invalidates are not affecting ring
> info. The downside being the overhead induced by checking whether the
> ring info are valid every time it is being used. I haven't prototyped
> this solution, but I expected the performance regression to be a
> blocker.
> 
> 4. In SET_VRING_ENABLE, don't enable the ring if needed entries are not in
> IOTLB cache. Just send the IOTLB misses without reply-ack flag and postpone
> enable when handling IOTLB updates. It will be a little more complex
> solution than current one, but it may be the less impacting compared to the
> other 3 options.
> 
> 
> Thinking again, maybe trying solution would be worth the effort, and could
> be extended also to disable the rings when receiving invalidates
> that affect rings info.
> 
> What do you think?

I'm fine with 3 or 4 generally. But pls note that if the ring crosses a
page boundary (e.g. ring size > page size) and when not using
hugetlbfs, there is no guarantee a single DMA address covers the
whole ring.

> > 
> > > 
> > > Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> > > ---
> > > v2:
> > >   - Revert back to existing behaviour, i.e. only send IOTLB updates
> > > at ring enablement time, not at ring address setting time (mst).
> > >   - Extend IOTLB misses to all ring addresses, not only used ring.
> > > 
> > >   hw/virtio/vhost.c | 8 ++++++--
> > >   1 file changed, 6 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > > index 6eddb09..7867034 100644
> > > --- a/hw/virtio/vhost.c
> > > +++ b/hw/virtio/vhost.c
> > > @@ -1552,11 +1552,15 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
> > >       if (vhost_dev_has_iommu(hdev)) {
> > >           hdev->vhost_ops->vhost_set_iotlb_callback(hdev, true);
> > > -        /* Update used ring information for IOTLB to work correctly,
> > > -         * vhost-kernel code requires for this.*/
> > > +        /*
> > > +         * Update rings information for IOTLB to work correctly,
> > > +         * vhost-kernel and vhost-user codes require for this.
> > 
> > Better just say "Update ring info for vhost iotlb."
> > 
> > The rest isn't really informative.
> 
> Ok.
> 
> > 
> > 
> > 
> > > +         */
> > >           for (i = 0; i < hdev->nvqs; ++i) {
> > >               struct vhost_virtqueue *vq = hdev->vqs + i;
> > >               vhost_device_iotlb_miss(hdev, vq->used_phys, true);
> > > +            vhost_device_iotlb_miss(hdev, vq->desc_phys, true);
> > > +            vhost_device_iotlb_miss(hdev, vq->avail_phys, true);
> > 
> > So I don't remember why does vhost in kernel want miss on used
> > at start time.
> > 
> > Jason, could you comment on this please?
> > 
> > 
> > 
> > >           }
> > >       }
> > >       return 0;
> > > -- 
> > > 2.9.4
> 
> Thanks,
> Maxime