Re: [Xen-devel] PCI passthrough issue

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Wed, 2011-02-02 at 15:38 +0000, Konrad Rzeszutek Wilk wrote:
> > > Ping tests work whatever can be packet size.
> > > 
> > > If I understood well what you explain to me, it's now clear that the
> > > problem is somewhat Xen related, isn't it ?
> > 
> > It certainly seems that way. I'm not 100% sure that swiotlb=force will
> > have actually made the driver use the swiotlb, it may just have forced
> 
> It should have.
> 
> > the swiotlb to be allocated. Konrad?
> 
> Both. It would allocate it and force all DMA operations to go through
> the bounce buffer.
> 
> Let me take a look at the driver itself. Ah, so I had a similar card: 3c59x
> which hit the same type of failure way back in 2.6.31 time. It had the
> pci_dma_sync_single_for_cpu(.. PCI_DMA_FROMDEVICE) turned around so it would 
> never
> copy properly. But in my tree (2.6.38) the sky driver looks to be doing just
> right.. It has PCI_DMA_FROMDEVICE..
> 
> Well this looks odd:
> 
> 2330         skb = netdev_alloc_skb_ip_align(sky2->netdev, length);
> 2331         if (likely(skb)) {
> 2332                 pci_dma_sync_single_for_cpu(sky2->hw->pdev, 
> re->data_addr,
> 2333                                             length, PCI_DMA_FROMDEVICE); 
> =======>             skb_copy_from_linear_data(re->skb, skb->data, length); 
> <====
> 2335                 skb->ip_summed = re->skb->ip_summed;
> 2336                 skb->csum = re->skb->csum;
> 2337                 pci_dma_sync_single_for_device(sky2->hw->pdev, 
> re->data_addr,
> 2338                                                length, 
> PCI_DMA_FROMDEVICE); 
> 2339                 re->skb->ip_summed = CHECKSUM_NONE;
> 2340                 skb_put(skb, length);
> 2341         }
> 
> It copies from skb->data (just allocated) to re->skb (seems that the 
> re->skb->data
> has been earlier DMA-mapped).
> 
> But there is no data in skb->data as we just allocated it? Shouldn't this
> be the other way around? Like this:

skb_copy_from_linear_data's arguments are the other way round to what
you may be expecting:
static inline void skb_copy_from_linear_data(const struct sk_buff *skb,
                                             void *to,
                                             const unsigned int len)

So the above line is correctly copying from re->skb->data to skb->data
not the other way round.

I stared at that exact line for ages this morning ;-)

Ian.


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