In Xen environment, if Xen-swiotlb is enabled, ixgbe driver
could possibly allocate a page, DMA memory buffer, for the first
fragment which is not suitable for Xen-swiotlb to do DMA operations.
Xen-swiotlb have to internally allocate another page for doing DMA
operations. This mechanism requires syncing the data from the internal
page to the page which ixgbe sends to upper network stack. However,
since commit f3213d93 ("ixgbe: Update driver to make use of DMA
attributes in Rx path"), the unmap operation is performed with
DMA_ATTR_SKIP_CPU_SYNC. As a result, the sync is not performed.
Since the sync isn't performed, the upper network stack could receive
a incomplete network packet. By incomplete, it means the linear data
on the first fragment(between skb->head and skb->end) is invalid. So
we have to copy the data from the internal xen-swiotlb page to the page
which ixgbe sends to upper network stack through the sync operation.

More details from Alexander Duyck:
Specifically since we are mapping the frame with
DMA_ATTR_SKIP_CPU_SYNC we have to unmap with that as well. As a result
a sync is not performed on an unmap and must be done manually as we
skipped it for the first frag. As such we need to always sync before
possibly performing a page unmap operation.

Fixes: f3213d93 ("ixgbe: Update driver to make use of DMA attributes in Rx path")
Signed-off-by: Firo Yang <firo.yang@suse.com>
Reviewed-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>