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SubjectRe: [PATCH v2 1/1] ixgbe: sync the first fragment unconditionally
Date
The 08/07/2019 15:56, Jacob Wen wrote:
> I think the description is not correct. Consider using something like below.
Thank you for comments.

>
> In Xen environment, due to memory fragmentation ixgbe may allocate a 'DMA'
> buffer with pages that are not physically contiguous.
Actually, I didn't look into the reason why ixgbe got a DMA buffer which
was mapped to Xen-swiotlb area.

But I don't think this issue relates to phsical memory contiguity because, in
our case, one ixgbe_rx_buffer only associates at most one page.

If you take a look at the related code, you will find there are several reasons
for mapping a DMA buffer to Xen-swiotlb area:
static dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
*/
if (dma_capable(dev, dev_addr, size) &&
!range_straddles_page_boundary(phys, size) &&
!xen_arch_need_swiotlb(dev, phys, dev_addr) &&
swiotlb_force != SWIOTLB_FORCE)
goto done;

// Firo
>
> A NIC doesn't support directly write such buffer. So xen-swiotlb would use
> the pages, which are physically contiguous, from the swiotlb buffer for the
> NIC.
>
> The unmap operation is used to copy the swiotlb buffer to the pages that are
> allocated by ixgbe.
>
> On 8/7/19 10:49 AM, Firo Yang wrote:
> > 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. It requires syncing between those two pages. 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.
> >
> > To fix this problem, 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@linux.intel.com>
> > Signed-off-by: Firo Yang <firo.yang@suse.com>
> > ---
> >
> > 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(-)
> >
> > diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > index cbaf712d6529..200de9838096 100644
> > --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > @@ -1825,13 +1825,7 @@ static void ixgbe_pull_tail(struct ixgbe_ring *rx_ring,
> > static void ixgbe_dma_sync_frag(struct ixgbe_ring *rx_ring,
> > struct sk_buff *skb)
> > {
> > - /* if the page was released unmap it, else just sync our portion */
> > - if (unlikely(IXGBE_CB(skb)->page_released)) {
> > - dma_unmap_page_attrs(rx_ring->dev, IXGBE_CB(skb)->dma,
> > - ixgbe_rx_pg_size(rx_ring),
> > - DMA_FROM_DEVICE,
> > - IXGBE_RX_DMA_ATTR);
> > - } else if (ring_uses_build_skb(rx_ring)) {
> > + if (ring_uses_build_skb(rx_ring)) {
> > unsigned long offset = (unsigned long)(skb->data) & ~PAGE_MASK;
> > dma_sync_single_range_for_cpu(rx_ring->dev,
> > @@ -1848,6 +1842,14 @@ static void ixgbe_dma_sync_frag(struct ixgbe_ring *rx_ring,
> > skb_frag_size(frag),
> > DMA_FROM_DEVICE);
> > }
> > +
> > + /* If the page was released, just unmap it. */
> > + if (unlikely(IXGBE_CB(skb)->page_released)) {
> > + dma_unmap_page_attrs(rx_ring->dev, IXGBE_CB(skb)->dma,
> > + ixgbe_rx_pg_size(rx_ring),
> > + DMA_FROM_DEVICE,
> > + IXGBE_RX_DMA_ATTR);
> > + }
> > }
> > /**
>
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