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

[Laurent Vivier]

On 13/01/2016 10:06, Greg Kurz wrote:
> 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

OK, you're right, I was studying sources without commit:

052bd52 net: don't set native endianness

Sorry for the noise...

> 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,
>>>
>>>   
>>
>