Re: [Qemu-devel] [RFC v2 8/8] virtio: guest driver reload for vhost-net

[Michael S. Tsirkin]

On Fri, Sep 21, 2018 at 09:30:26AM +0200, Maxime Coquelin wrote:
> 
> 
> On 09/21/2018 04:33 AM, Jason Wang wrote:
> > 
> > 
> > On 2018年09月21日 04:39, Maxime Coquelin wrote:
> > > Hi Wei, Jason,
> > > 
> > > On 06/19/2018 09:53 AM, Wei Xu wrote:
> > > > On Wed, Jun 06, 2018 at 11:48:19AM +0800, Jason Wang wrote:
> > > > > 
> > > > > 
> > > > > On 2018年06月06日 03:08, address@hidden wrote:
> > > > > > From: Wei Xu <address@hidden>
> > > > > > 
> > > > > > last_avail, avail_wrap_count, used_idx and used_wrap_count are
> > > > > > needed to support vhost-net backend, all these are either 16 or
> > > > > > bool variables, since state.num is 64bit wide, so here it is
> > > > > > possible to put them to the 'num' without introducing a new case
> > > > > > while handling ioctl.
> > > > > > 
> > > > > > Unload/Reload test has been done successfully with a
> > > > > > patch in vhost kernel.
> > > > > 
> > > > > You need a patch to enable vhost.
> > > > > 
> > > > > And I think you can only do it for vhost-kenrel now since vhost-user
> > > > > protocol needs some extension I believe.
> > > > 
> > > > OK.
> > > > 
> > > > > 
> > > > > > 
> > > > > > Signed-off-by: Wei Xu <address@hidden>
> > > > > > ---
> > > > > >   hw/virtio/virtio.c | 42 ++++++++++++++++++++++++++++++++++--------
> > > > > >   1 file changed, 34 insertions(+), 8 deletions(-)
> > > > > > 
> > > > > > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > > > > > index 4543974..153f6d7 100644
> > > > > > --- a/hw/virtio/virtio.c
> > > > > > +++ b/hw/virtio/virtio.c
> > > > > > @@ -2862,33 +2862,59 @@ hwaddr
> > > > > > virtio_queue_get_used_size(VirtIODevice *vdev, int n)
> > > > > >       }
> > > > > >   }
> > > > > > -uint16_t virtio_queue_get_last_avail_idx(VirtIODevice *vdev, int n)
> > > > > > +uint64_t virtio_queue_get_last_avail_idx(VirtIODevice *vdev, int n)
> > > > > >   {
> > > > > > -    return vdev->vq[n].last_avail_idx;
> > > > > > +    uint64_t num;
> > > > > > +
> > > > > > +    num = vdev->vq[n].last_avail_idx;
> > > > > > +    if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> > > > > > +        num |= ((uint64_t)vdev->vq[n].avail_wrap_counter) << 16;
> > > > > > +        num |= ((uint64_t)vdev->vq[n].used_idx) << 32;
> > > > > > +        num |= ((uint64_t)vdev->vq[n].used_wrap_counter) << 48;
> > > > > 
> > > > > So s.num is 32bit, I don't think this can even work.
> > > > 
> > > > I mistakenly checked out s.num is 64bit, will add a new case in
> > > > next version.
> > > 
> > > Wouldn't be enough to just get/set avail_wrap_counter?
> > > Something like this so that it fits into 32 bits:
> > >     if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> > >         num |= ((uint64_t)vdev->vq[n].avail_wrap_counter) << 31;
> > >     }
> > > 
> > > 
> > > Regards,
> > > Maxime
> > 
> > Yes, actually, that's what I did for vhost kernel 
> > (https://github.com/jasowang/net/blob/packed-host/drivers/vhost/vhost.c#L1418)
> 
> Oh yes, I forgot on what I based my implementation on.
> Wei, Michael, do you agree with this? I will need to merge the Vhost
> patch in DPDK in the coming two weeks.
> 
> Regards,
> Maxime

It looks reasonable to me.

> > 
> > Thanks