Re: [Qemu-devel] [PATCH v2 1/2] virtio-net: use the backend cross-endian capabilities

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Thu, 12 Nov 2015 18:52:55 +0100
Cornelia Huck <cornelia.huck@de.ibm.com> wrote:

> On Mon, 09 Nov 2015 18:41:33 +0100
> Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:
> 
> > When running a fully emulated device in cross-endian conditions, including
> > a virtio 1.0 device offered to a big endian guest, we need to fix the vnet
> > headers. This is currently handled by the virtio_net_hdr_swap() function
> > in the core virtio-net code but it should actually be handled by the net
> > backend.
> > 
> > With this patch, virtio-net now tries to configure the backend to do the
> > endian fixing when the device starts. If the backend cannot support the
> > requested endiannes, we have to fall back on virtio_net_hdr_swap(): this
> > is recorded in the needs_vnet_hdr_swap flag.
> > 
> > The current vhost-net code also tries to configure net backends. This will
> > be no more needed and will be addressed in a subsequent patch.
> > 
> > Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> > ---
> > v2: - introduced virtio_net_needs_hdr_swap() to consolidate the logic in one
> >       place
> > ---
> >  hw/net/virtio-net.c            |   43 ++++++++++++++++++++++++++++++++++++++--
> >  include/hw/virtio/virtio-net.h |    1 +
> >  2 files changed, 42 insertions(+), 2 deletions(-)
> > 
> > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > index a877614e3e7a..a10f7a0b4d28 100644
> > --- a/hw/net/virtio-net.c
> > +++ b/hw/net/virtio-net.c
> > @@ -152,6 +152,33 @@ static void virtio_net_vhost_status(VirtIONet *n, uint8_t status)
> >      }
> >  }
> > 
> > +static int peer_has_vnet_hdr(VirtIONet *n);
> 
> Hm, you do not seem to use this?
> 

Oops I needed that in a earlier version of this patch and forgot to remove it...

> > +
> > +static void virtio_net_vnet_status(VirtIONet *n, uint8_t status)
> > +{
> > +    VirtIODevice *vdev = VIRTIO_DEVICE(n);
> > +    NetClientState *peer = qemu_get_queue(n->nic)->peer;
> > +
> > +    if (virtio_net_started(n, status)) {
> > +        int r;
> > +
> > +        if (virtio_is_big_endian(vdev)) {
> > +            r = qemu_set_vnet_be(peer, true);
> > +        } else {
> > +            r = qemu_set_vnet_le(peer, true);
> > +        }
> > +
> > +        n->needs_vnet_hdr_swap = !!r;
> > +    } else if (virtio_net_started(n, vdev->status) &&
> > +               !virtio_net_started(n, status)) {
> > +        if (virtio_is_big_endian(vdev)) {
> > +            qemu_set_vnet_be(peer, false);
> > +        } else {
> > +            qemu_set_vnet_le(peer, false);
> > +        }
> > +    }
> > +}
> > +
> >  static void virtio_net_set_status(struct VirtIODevice *vdev, uint8_t status)
> >  {
> >      VirtIONet *n = VIRTIO_NET(vdev);
> > @@ -159,6 +186,7 @@ static void virtio_net_set_status(struct VirtIODevice *vdev, uint8_t status)
> >      int i;
> >      uint8_t queue_status;
> > 
> > +    virtio_net_vnet_status(n, status);
> >      virtio_net_vhost_status(n, status);
> > 
> >      for (i = 0; i < n->max_queues; i++) {
> > @@ -949,6 +977,14 @@ static void work_around_broken_dhclient(struct virtio_net_hdr *hdr,
> >      }
> >  }
> > 
> > +static bool virtio_net_needs_hdr_swap(VirtIONet *n)
> > +{
> > +    /* virtio_needs_swap() is constant for fixed endian targets: call it
> > +     * first to filter them out without penalty.
> 
> What do you mean with 'constant' here? It still needs to retrieve the
> feature bit from the device, no?
> 

Yes the comment is inaccurate... With the "virtio: cross-endian helpers fixes"
series, virtio_needs_swap() indeed resolves to { return 0; } for fixed little
endian targets but we still need to check the feature bit when the target is big
endian.

If I drop the call to virtio_needs_swap(), all targets will have the same
penalty. If I keep it, fixed little endian targets have no penalty but fixed
big endian targets get an extra check and bi-endian targets get two extra checks...

Not sure what to decide...

> > +     */
> > +    return virtio_needs_swap(VIRTIO_DEVICE(n)) && n->needs_vnet_hdr_swap;
> > +}
> > +
> >  static void receive_header(VirtIONet *n, const struct iovec *iov, int iov_cnt,
> >                             const void *buf, size_t size)
> >  {
> > @@ -957,7 +993,10 @@ static void receive_header(VirtIONet *n, const struct iovec *iov, int iov_cnt,
> >          void *wbuf = (void *)buf;
> >          work_around_broken_dhclient(wbuf, wbuf + n->host_hdr_len,
> >                                      size - n->host_hdr_len);
> > -        virtio_net_hdr_swap(VIRTIO_DEVICE(n), wbuf);
> > +
> > +        if (virtio_net_needs_hdr_swap(n)) {
> > +            virtio_net_hdr_swap(VIRTIO_DEVICE(n), wbuf);
> > +        }
> >          iov_from_buf(iov, iov_cnt, 0, buf, sizeof(struct virtio_net_hdr));
> >      } else {
> >          struct virtio_net_hdr hdr = {
> > @@ -1167,7 +1206,7 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
> >                  error_report("virtio-net header incorrect");
> >                  exit(1);
> >              }
> > -            if (virtio_needs_swap(vdev)) {
> > +            if (virtio_net_needs_hdr_swap(n)) {
> >                  virtio_net_hdr_swap(vdev, (void *) &mhdr);
> >                  sg2[0].iov_base = &mhdr;
> >                  sg2[0].iov_len = n->guest_hdr_len;
> > diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
> > index f3cc25feca2b..27bc868fbc7d 100644
> > --- a/include/hw/virtio/virtio-net.h
> > +++ b/include/hw/virtio/virtio-net.h
> > @@ -94,6 +94,7 @@ typedef struct VirtIONet {
> >      uint64_t curr_guest_offloads;
> >      QEMUTimer *announce_timer;
> >      int announce_counter;
> > +    bool needs_vnet_hdr_swap;
> >  } VirtIONet;
> > 
> >  void virtio_net_set_netclient_name(VirtIONet *n, const char *name,
> > 
> > 
>