Re: [Qemu-devel] [PATCH V2] virtio-net: unbreak any layout

[Michael S. Tsirkin]

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: address@hidden
> >> Cc: address@hidden
> >> Signed-off-by: Jason Wang <address@hidden>
> >> ---
> >> 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