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

[Jason Wang]

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?

>
>
>> 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