Re: [PATCH 2/3] vdpa: load vlan configuration at NIC startup

[Eugenio Perez Martin]

On Fri, Sep 9, 2022 at 10:38 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Fri, Sep 09, 2022 at 10:01:16AM +0200, Eugenio Perez Martin wrote:
> > On Fri, Sep 9, 2022 at 8:40 AM Jason Wang <jasowang@redhat.com> wrote:
> > >
> > > On Fri, Sep 9, 2022 at 2:38 PM Jason Wang <jasowang@redhat.com> wrote:
> > > >
> > > > On Wed, Sep 7, 2022 at 12:36 AM Eugenio Pérez <eperezma@redhat.com> 
> > > > wrote:
> > > > >
> > > > > To have enabled vlans at device startup may happen in the destination 
> > > > > of
> > > > > a live migration, so this configuration must be restored.
> > > > >
> > > > > At this moment the code is not accessible, since SVQ refuses to start 
> > > > > if
> > > > > vlan feature is exposed by the device.
> > > > >
> > > > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > > > ---
> > > > >  net/vhost-vdpa.c | 46 ++++++++++++++++++++++++++++++++++++++++++++--
> > > > >  1 file changed, 44 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/net/vhost-vdpa.c b/net/vhost-vdpa.c
> > > > > index 4bc3fd01a8..ecbfd08eb9 100644
> > > > > --- a/net/vhost-vdpa.c
> > > > > +++ b/net/vhost-vdpa.c
> > > > > @@ -100,6 +100,8 @@ static const uint64_t vdpa_svq_device_features =
> > > > >      BIT_ULL(VIRTIO_NET_F_RSC_EXT) |
> > > > >      BIT_ULL(VIRTIO_NET_F_STANDBY);
> > > > >
> > > > > +#define MAX_VLAN    (1 << 12)   /* Per 802.1Q definition */
> > > > > +
> > > > >  VHostNetState *vhost_vdpa_get_vhost_net(NetClientState *nc)
> > > > >  {
> > > > >      VhostVDPAState *s = DO_UPCAST(VhostVDPAState, nc, nc);
> > > > > @@ -423,6 +425,47 @@ static int vhost_vdpa_net_load_mq(VhostVDPAState 
> > > > > *s,
> > > > >      return *s->status != VIRTIO_NET_OK;
> > > > >  }
> > > > >
> > > > > +static int vhost_vdpa_net_load_single_vlan(VhostVDPAState *s,
> > > > > +                                           const VirtIONet *n,
> > > > > +                                           uint16_t vid)
> > > > > +{
> > > > > +    ssize_t dev_written = vhost_vdpa_net_load_cmd(s, 
> > > > > VIRTIO_NET_CTRL_VLAN,
> > > > > +                                                  
> > > > > VIRTIO_NET_CTRL_VLAN_ADD,
> > > > > +                                                  &vid, sizeof(vid));
> > > > > +    if (unlikely(dev_written < 0)) {
> > > > > +        return dev_written;
> > > > > +    }
> > > > > +
> > > > > +    if (unlikely(*s->status != VIRTIO_NET_OK)) {
> > > > > +        return -EINVAL;
> > > > > +    }
> > > > > +
> > > > > +    return 0;
> > > > > +}
> > > > > +
> > > > > +static int vhost_vdpa_net_load_vlan(VhostVDPAState *s,
> > > > > +                                    const VirtIONet *n)
> > > > > +{
> > > > > +    uint64_t features = n->parent_obj.guest_features;
> > > > > +
> > > > > +    if (!(features & BIT_ULL(VIRTIO_NET_F_CTRL_VLAN))) {
> > > > > +        return 0;
> > > > > +    }
> > > > > +
> > > > > +    for (int i = 0; i < MAX_VLAN >> 5; i++) {
> > > > > +        for (int j = 0; n->vlans[i] && j <= 0x1f; j++) {
> > > > > +            if (n->vlans[i] & (1U << j)) {
> > > > > +                int r = vhost_vdpa_net_load_single_vlan(s, n, (i << 
> > > > > 5) + j);
> > > >
> > > > This seems to cause a lot of latency if the driver has a lot of vlans.
> > > >
> > > > I wonder if it's simply to let all vlan traffic go by disabling
> > > > CTRL_VLAN feature at vDPA layer.
> > >
> >
> > The guest will not be able to recover that vlan configuration.
> >
> > > Another idea is to extend the spec to allow us to accept a bitmap of
> > > the vlan ids via a single command, then we will be fine.
> > >
> >
> > I'm not sure if adding more ways to configure something is the answer,
> > but I'd be ok to implement it.
> >
> > Another idea is to allow the sending of many CVQ commands in parallel.
> > It shouldn't be very hard to achieve using exposed buffers as ring
> > buffers, and it will short down the start of the devices with many
> > features.
>
> This would seem like a reasonable second step.  A first step would be to
> measure the overhead to make sure there's actually a need to optimize
> this.
>

I totally agree.

The processing time of each command is limited by the device. Taking
this to the extreme, we could achieve almost instantaneous
configuration for vdpa_sim_net, but a new device could take forever to
configure each mac.

So I think it should be considered as an optimization on top too, and
apply only if we see the initialization is slow on any devices.

Thanks!