Found an issue with the IDPF driver when SWIOTLB is enabled. The issue
results in empty headers for packets that hit the split queue workaround
path. It's caused by a spurious sync in that path. The header is synced
from the SWIOTLB even when the header was shoved into the payload.

I cooked up a sample patch, but I'm not an expert in this driver, so I have
no idea if it's the right solution. It did allow my QEMU VM to boot with a
superficially functional passed-through IDPF NIC and SWIOTLB=force.

The patch was written against COS's 6.12, so I assume that it will not
apply cleanly elsewhere, but I figured a wrong sample patch was better than
a long paragraph describing the same thing. My read of more recent kernels
is that this problem is still present, but could be mistaken.

Steve Rutherford (1):
  Fix header clobber in IDPF with SWIOTLB enabled

 drivers/net/ethernet/intel/idpf/idpf_txrx.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

-- 
2.53.0.473.g4a7958ca14-goog