Re: [Qemu-devel] [PATCH 1/6] virtio-net: use the backend cross-endian capabilities

[Greg Kurz]

On Fri, 8 Jan 2016 16:25:18 +0100
Laurent Vivier <address@hidden> wrote:

> 
> 
> On 08/01/2016 15:19, Greg Kurz wrote:
> > On Thu, 7 Jan 2016 19:32:37 +0100
> > Laurent Vivier <address@hidden> wrote:
> >>
> > 
> > Sorry for the late answer to this one, I got diverted :)
> > 
> >>
> >> On 07/01/2016 12:32, 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. 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, to be used in the TX and RX
> >>> paths.
> >>>
> >>> 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 <address@hidden>
> >>> ---
> >>>  hw/net/virtio-net.c            |   33 +++++++++++++++++++++++++++++++--
> >>>  include/hw/virtio/virtio-net.h |    1 +
> >>>  2 files changed, 32 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> >>> index a877614e3e7a..d4cc94ea5e55 100644
> >>> --- a/hw/net/virtio-net.c
> >>> +++ b/hw/net/virtio-net.c
> >>> @@ -152,6 +152,31 @@ 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;
> >>> +
> >>> +        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);
> >>> +        }
> >>> +    }
> >>> +}
> >>
> >> Could you explain why check 'virtio_net_started(n, status)' and then
> >> 'virtio_net_started(n, vdev->status)' ?
> >>
> > 
> > Before using the device, we need to inform the network backend about
> > the endianness to use when parsing vnet headers. We do this when the
> > driver activates the device (DRIVER_OK). This is the first check.
> > 
> > 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. We do this
> > when the driver deactivates the device (no DRIVER_OK). The second check
> > ensures the device was active before: if we don't check that, the 'else'
> > branch would be executed each time the driver updates the status with
> > something not containing DRIVER_OK... :-\
> > 
> >> Why qemu_set_vnet_[bl]e() use "true" in the first case and "false" in
> >> the second case ?
> >>
> > 
> > "true" tells the backend to enforce the corresponding endianness.
> > "false" tells the backed to reset to the default (guest native endianness).
> > 
> >> Why don't you store the result (r) in the second case ?
> >>
> > 
> > Because @needs_vnet_hdr_swap is only being used when the device is active.
> > 
> > Thank you for the time you spent on reviewing this series !
> 
> Thank you for the details, it's clear now.
> Perhaps this can be added in the commit log or in some comments ?
> 

I realized when writing the mail that this is non-trivial indeed. I'm
currently adding comments and updating the changelog :)

> > Bonne Annee !
> 
> Bonne Année ;)
> Laurent
> 
> > --
> > Greg
> > 
> >>>  static void virtio_net_set_status(struct VirtIODevice *vdev, uint8_t 
> >>> status)
> >>>  {
> >>>      VirtIONet *n = VIRTIO_NET(vdev);
> >>> @@ -159,6 +184,7 @@ 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 +983,10 @@ 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);
> >>> +        }
> >>>          iov_from_buf(iov, iov_cnt, 0, buf, sizeof(struct 
> >>> virtio_net_hdr));
> >>>      } else {
> >>>          struct virtio_net_hdr hdr = {
> >>> @@ -1167,7 +1196,7 @@ 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-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,
> >>>
> >>>
> >>
> > 
>