RE: [PATCH v3 1/1] ixgbe: sync the first fragment unconditionally

[Bowers, AndrewX]

> -----Original Message-----
> From: Intel-wired-lan [mailto:intel-wired-lan-bounces@xxxxxxxxxx] On
> Behalf Of Firo Yang
> Sent: Wednesday, August 7, 2019 9:04 PM
> To: netdev@xxxxxxxxxxxxxxx
> Cc: maciejromanfijalkowski@xxxxxxxxx; Firo Yang <firo.yang@xxxxxxxx>;
> linux-kernel@xxxxxxxxxxxxxxx; intel-wired-lan@xxxxxxxxxxxxxxxx;
> jian.w.wen@xxxxxxxxxx; alexander.h.duyck@xxxxxxxxxxxxxxx;
> davem@xxxxxxxxxxxxx
> Subject: [Intel-wired-lan] [PATCH v3 1/1] ixgbe: sync the first fragment
> unconditionally
> 
> 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 f3213d932173 ("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: f3213d932173 ("ixgbe: Update driver to make use of DMA attributes in
> Rx path")
> Reviewed-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> Signed-off-by: Firo Yang <firo.yang@xxxxxxxx>
> ---
> Changes from v2:
>  * Added details on the problem caused by skipping the sync.
>  * Added more explanation from Alexander Duyck.
> 
> Changes from v1:
>  * Imporved the patch description.
>  * Added Reviewed-by: and Fixes: as suggested by Alexander Duyck.
> 
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)

Tested-by: Andrew Bowers <andrewx.bowers@xxxxxxxxx>