Re: [Qemu-devel] [PATCH v2 1/5] virtio-net: use the backend cross-endian capabilities

[Greg Kurz]

On Tue, 12 Jan 2016 18:24:28 +0100
Laurent Vivier <address@hidden> wrote:

> On 11/01/2016 17:12, Greg Kurz 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 (i.e. drivers sets the CONFIG_OK bit).
> > If the backend cannot support the requested endiannes, we have to fallback
> > onto virtio_net_hdr_swap(): this is recorded in the needs_vnet_hdr_swap 
> > flag,
> > to be used in the TX and RX paths.
> > 
> > Note that we reset the backend to the default behaviour (guest native
> > endianness) when the device stops (i.e. device status had CONFIG_OK bit and
> > driver unsets it). This is needed, with the linux tap backend at least,
> > otherwise the guest may loose network connectivity if rebooted into a
> > different endianness.
> > 
> > The current vhost-net code also tries to configure net backends. This will
> > be no more needed and will be reverted in a subsequent patch.
> > 
> > Signed-off-by: Greg Kurz <address@hidden>
> > ---
> > v2:
> > - dropped useless check in the 'else if' branch in virtio_net_vnet_status()
> > - merged virtio_net_vhost_status() change from patch 2
> > - use semicolon in "backend does no support..." error message
> > - merged patch 3 (drop the virtio_needs_swap() helper)
> > - provided some more details in changelog and comments
> > ---
> >  hw/net/virtio-net.c               |   49 
> > +++++++++++++++++++++++++++++++++++--
> >  include/hw/virtio/virtio-access.h |    9 -------
> >  include/hw/virtio/virtio-net.h    |    1 +
> >  3 files changed, 48 insertions(+), 11 deletions(-)
> > 
> > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > index a877614e3e7a..497fb7119a08 100644
> > --- a/hw/net/virtio-net.c
> > +++ b/hw/net/virtio-net.c
> > @@ -128,6 +128,13 @@ static void virtio_net_vhost_status(VirtIONet *n, 
> > uint8_t status)
> >      if (!n->vhost_started) {
> >          int r, i;
> >  
> > +        if (n->needs_vnet_hdr_swap) {
> > +            error_report("backend does not support %s vnet headers; "
> > +                         "falling back on userspace virtio",
> > +                         virtio_is_big_endian(vdev) ? "BE" : "LE");
> > +            return;
> > +        }
> > +
> >          /* Any packets outstanding? Purge them to avoid touching rings
> >           * when vhost is running.
> >           */
> > @@ -152,6 +159,40 @@ static void virtio_net_vhost_status(VirtIONet *n, 
> > uint8_t status)
> >      }
> >  }
> >  
> > +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;
> > +
> > +        /* Before using the device, we tell the network backend about the
> > +         * endianness to use when parsing vnet headers. If the backend 
> > can't
> > +         * do it, we fallback onto fixing the headers in the core 
> > virtio-net
> > +         * code.
> > +         */
> > +        if (virtio_is_big_endian(vdev)) {
> > +            r = qemu_set_vnet_be(peer, true);
> > +        } else {
> > +            r = qemu_set_vnet_le(peer, true);
> > +        }  
> 
> If endianess of the guest and the virtio device is the same, but r is <
> 0 (-ENOSYS or -EINVAL) you will badly swap header (and disable vhost).
> 

This can only happen if the endianness of the host is not the same as the
endianness of the device. In this case (we usually call cross-endian) the
vnet headers must be byteswapped but the backend cannot handle it. This
has two consequences:
- vhost cannot be used since it requires the backend to support cross-endian
  vnet headers, so we fallback onto full emulation in QEMU
- the emulation code must byteswap vnet headers

> I think you need something like this to fall back to the old method:
> 
>         if (r < 0) {
> #ifdef HOST_WORDS_BIGENDIAN
>             r = virtio_access_is_big_endian(vdev) ? false : true;
> #else
>             r = virtio_access_is_big_endian(vdev) ? true : false;
> #endif
>         }
> 
> 
> But...
> 
> > +        n->needs_vnet_hdr_swap = !!r;
> > +    } else if (virtio_net_started(n, vdev->status)) {
> > +        /* After using the device, we need to reset the network backend to
> > +         * the default (guest native endianness), otherwise the guest may
> > +         * loose network connectivity if it is rebooted into a different
> > +         * endianness.
> > +         */
> > +        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 +200,7 @@ 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++) {
> > @@ -957,7 +999,10 @@ 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 (n->needs_vnet_hdr_swap) {
> > +            virtio_net_hdr_swap(VIRTIO_DEVICE(n), wbuf);
> > +        }  
> 
> ... this will change the behavior here, as before it was not
> conditional. Why ?
> 

This is what this patch is all about as described above.

> >          iov_from_buf(iov, iov_cnt, 0, buf, sizeof(struct virtio_net_hdr));
> >      } else {
> >          struct virtio_net_hdr hdr = {
> > @@ -1167,7 +1212,7 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
> >                  error_report("virtio-net header incorrect");
> >                  exit(1);
> >              }
> > -            if (virtio_needs_swap(vdev)) {
> > +            if (n->needs_vnet_hdr_swap) {
> >                  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-access.h 
> > b/include/hw/virtio/virtio-access.h
> > index 8aec843c8ff3..a01fff2e51d7 100644
> > --- a/include/hw/virtio/virtio-access.h
> > +++ b/include/hw/virtio/virtio-access.h
> > @@ -143,15 +143,6 @@ static inline uint64_t virtio_ldq_p(VirtIODevice 
> > *vdev, const void *ptr)
> >      }
> >  }
> >  
> > -static inline bool virtio_needs_swap(VirtIODevice *vdev)
> > -{
> > -#ifdef HOST_WORDS_BIGENDIAN
> > -    return virtio_access_is_big_endian(vdev) ? false : true;
> > -#else
> > -    return virtio_access_is_big_endian(vdev) ? true : false;
> > -#endif
> > -}
> > -
> >  static inline uint16_t virtio_tswap16(VirtIODevice *vdev, uint16_t s)
> >  {
> >  #ifdef HOST_WORDS_BIGENDIAN
> > 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,
> > 
> >   
>