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

[Maxime Coquelin]

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

> Wei
>
> > Thanks
> >
> > > +    }
> > > +
> > > +    return num;
> > >   }
> > > -void virtio_queue_set_last_avail_idx(VirtIODevice *vdev, int n, uint16_t idx)
> > > +void virtio_queue_set_last_avail_idx(VirtIODevice *vdev, int n, uint64_t num)
> > >   {
> > > -    vdev->vq[n].last_avail_idx = idx;
> > > -    vdev->vq[n].shadow_avail_idx = idx;
> > > +    vdev->vq[n].shadow_avail_idx = vdev->vq[n].last_avail_idx = 
> > > (uint16_t)(num);
> > > +
> > > +    if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> > > +        vdev->vq[n].avail_wrap_counter = (uint16_t)(num >> 16);
> > > +        vdev->vq[n].used_idx = (uint16_t)(num >> 32);
> > > +        vdev->vq[n].used_wrap_counter = (uint16_t)(num >> 48);
> > > +    }
> > >   }
> > >   void virtio_queue_restore_last_avail_idx(VirtIODevice *vdev, int n)
> > >   {
> > >       rcu_read_lock();
> > > -    if (vdev->vq[n].vring.desc) {
> > > +    if (!vdev->vq[n].vring.desc) {
> > > +        goto out;
> > > +    }
> > > +
> > > +    if (!virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> > >           vdev->vq[n].last_avail_idx = vring_used_idx(&vdev->vq[n]);
> > > -        vdev->vq[n].shadow_avail_idx = vdev->vq[n].last_avail_idx;
> > >       }
> > > +    vdev->vq[n].shadow_avail_idx = vdev->vq[n].last_avail_idx;
> > > +
> > > +out:
> > >       rcu_read_unlock();
> > >   }
> > >   void virtio_queue_update_used_idx(VirtIODevice *vdev, int n)
> > >   {
> > >       rcu_read_lock();
> > > -    if (vdev->vq[n].vring.desc) {
> > > +    if (!vdev->vq[n].vring.desc) {
> > > +        goto out;
> > > +    }
> > > +
> > > +    if (!virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> > >           vdev->vq[n].used_idx = vring_used_idx(&vdev->vq[n]);
> > >       }
> > > +
> > > +out:
> > >       rcu_read_unlock();
> > >   }