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

[Laurent Vivier <lvivier@redhat.com>]

On 08/01/2016 15:19, Greg Kurz wrote:
> On Thu, 7 Jan 2016 19:32:37 +0100
> Laurent Vivier <lvivier@redhat.com> 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 <gkurz@linux.vnet.ibm.com>
>>> ---
>>>  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 ?

> 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,
>>>
>>>
>>
>