netdev - Re: [PATCH v2 1/1] ixgbe: sync the first fragment unconditionally

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Message-ID: <20190807083831.GA6811@linux-6qg8>
Date:   Wed, 7 Aug 2019 08:38:43 +0000
From:   Firo Yang <firo.yang@...e.com>
To:     Jacob Wen <jian.w.wen@...cle.com>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "jeffrey.t.kirsher@...el.com" <jeffrey.t.kirsher@...el.com>,
        "alexander.h.duyck@...ux.intel.com" 
        <alexander.h.duyck@...ux.intel.com>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] ixgbe: sync the first fragment unconditionally

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@...ux.intel.com>
> > Signed-off-by: Firo Yang <firo.yang@...e.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);
> > +	}
> >   }
> >   /**
>