On Thu, Jul 16, 2015 at 01:54:38PM +0800, Jason Wang wrote: > > > On 07/15/2015 07:23 PM, Michael S. Tsirkin wrote: > > On Wed, Jul 15, 2015 at 03:56:07PM +0800, Jason Wang wrote: > >> Commit 032a74a1c0fcdd5fd1c69e56126b4c857ee36611 > >> ("virtio-net: byteswap virtio-net header") breaks any layout by > >> requiring out_sg[0].iov_len >= n->guest_hdr_len. Fixing this by > >> copying header to temporary buffer if swap is needed, and then use > >> this buffer as part of out_sg. > >> > >> Fixes 032a74a1c0fcdd5fd1c69e56126b4c857ee36611 > >> ("virtio-net: byteswap virtio-net header") > >> Cc: qemu-sta...@nongnu.org > >> Cc: c...@fr.ibm.com > >> Signed-off-by: Jason Wang <jasow...@redhat.com> > >> --- > >> Changes from V1: > >> - avoid header copying if there's no need to do header swap > >> - don't write the header back > >> --- > >> hw/net/virtio-net.c | 17 ++++++++++++++--- > >> include/hw/virtio/virtio-access.h | 9 +++++++++ > >> 2 files changed, 23 insertions(+), 3 deletions(-) > >> > >> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c > >> index e3c2db3..12322bd 100644 > >> --- a/hw/net/virtio-net.c > >> +++ b/hw/net/virtio-net.c > >> @@ -1142,7 +1142,8 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q) > >> ssize_t ret, len; > >> unsigned int out_num = elem.out_num; > >> struct iovec *out_sg = &elem.out_sg[0]; > >> - struct iovec sg[VIRTQUEUE_MAX_SIZE]; > >> + struct iovec sg[VIRTQUEUE_MAX_SIZE], sg2[VIRTQUEUE_MAX_SIZE]; > >> + struct virtio_net_hdr hdr; > >> > >> if (out_num < 1) { > >> error_report("virtio-net header not in first element"); > >> @@ -1150,11 +1151,21 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue > >> *q) > >> } > >> > >> if (n->has_vnet_hdr) { > >> - if (out_sg[0].iov_len < n->guest_hdr_len) { > >> + if (iov_size(out_sg, out_num) < n->guest_hdr_len) { > >> error_report("virtio-net header incorrect"); > >> exit(1); > >> } > >> - virtio_net_hdr_swap(vdev, (void *) out_sg[0].iov_base); > >> + if (virtio_needs_swap(vdev)) { > >> + iov_to_buf(out_sg, out_num, 0, &hdr, sizeof(hdr)); > >> + virtio_net_hdr_swap(vdev, (void *) &hdr); > >> + sg2[0].iov_base = &hdr; > >> + sg2[0].iov_len = sizeof(hdr); > >> + out_num = iov_copy(&sg2[1], ARRAY_SIZE(sg2) - 1, > >> + out_sg, out_num, > >> + sizeof(hdr), -1); > >> + out_num += 1; > >> + out_sg = sg2; > >> + } > >> } > >> > >> /* > > I guess this works but iov cpy is pretty expensive. > > > > - When using the tun backend, we can just set VNETBE/VNETLE > > correctly and avoid swaps in userspace. > > It's easy to probe for - why don't we do this? > > We need to fix stable branches first. And if we do this for 2.4, if > running on a older kernel which does not support VNETBE/VNETLE, do we > want to fail the initialization? If yes, it breaks backward > compatibility. If no, we still need the swap here. > > > > > - OTOH when not using the tun backend, offloads are > > generally disabled, so the swap isn't needed > > since header is going to be all 0s. > > I agree it's better to check backend (e.g host_hdr_len) before trying to > do the swap. > > > > > - what's left is old kernels when using tun. > > how about probing for that explicitly? > > In other words, virtio_net_needs_swap as opposed to > > the generic virtio_needs_swap. > > So we still need swap anyway. How about apply this patch first for > stable and 2.4 and probe the VNETBE/VNETLE explicitly on top for 2.5?
I'm fine with it being a patch on top, but I think it's preferable to use VNETBE/VNETLE even in 2.4 if that's available. > > > > > >> diff --git a/include/hw/virtio/virtio-access.h > >> b/include/hw/virtio/virtio-access.h > >> index cee5dd7..1ec1dfd 100644 > >> --- a/include/hw/virtio/virtio-access.h > >> +++ b/include/hw/virtio/virtio-access.h > >> @@ -143,6 +143,15 @@ 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 > >> -- > >> 2.1.4