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

[Greg Kurz]

On Wed, 13 Jan 2016 10:53:11 +0100
Laurent Vivier <address@hidden> wrote:

> 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...
> 

No problem. Thank you again for your time !


> > 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,
> >>>
> >>>     
> >>  
> >   
>