[Xen-devel] Dom0 physical networking/swiotlb/something issue in 3.7-rc1

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

Hi Konrad,

The following patch causes fairly large packet loss when transmitting
from dom0 to the physical network, at least with my tg3 hardware, but I
assume it can impact anything which uses this interface.

I suspect that the issue is that the compound pages allocated in this
way are not backed by contiguous mfns and so things fall apart when the
driver tries to do DMA.

However I don't understand why the swiotlb is not fixing this up
successfully? The tg3 driver seems to use pci_map_single on this data.
Any thoughts? Perhaps the swiotlb (either generically or in the Xen
backend) doesn't correctly handle compound pages?

Ideally we would also fix this at the point of allocation to avoid the
bouncing -- I suppose that would involve using the DMA API in
netdev_alloc_frag?

We have a, sort of, similar situation in the block layer which is solved
via BIOVEC_PHYS_MERGEABLE. Sadly I don't think anything similar can
easily be retrofitted to the net drivers without changing every single
one.

Ian.

commit 69b08f62e17439ee3d436faf0b9a7ca6fffb78db
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Wed Sep 26 06:46:57 2012 +0000

    net: use bigger pages in __netdev_alloc_frag
    
    We currently use percpu order-0 pages in __netdev_alloc_frag
    to deliver fragments used by __netdev_alloc_skb()
    
    Depending on NIC driver and arch being 32 or 64 bit, it allows a page to
    be split in several fragments (between 1 and 8), assuming PAGE_SIZE=4096
    
    Switching to bigger pages (32768 bytes for PAGE_SIZE=4096 case) allows :
    
    - Better filling of space (the ending hole overhead is less an issue)
    
    - Less calls to page allocator or accesses to page->_count
    
    - Could allow struct skb_shared_info futures changes without major
      performance impact.
    
    This patch implements a transparent fallback to smaller
    pages in case of memory pressure.
    
    It also uses a standard "struct page_frag" instead of a custom one.
    
    Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxx>
    Cc: Benjamin LaHaise <bcrl@xxxxxxxxx>
    Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>




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