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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-11-09 at 09:03 +0000, Jan Beulich wrote:
> >>> On 12.10.12 at 12:28, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > 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.
> 
> Has this seen any sort of resolution yet? Despite having forced
> NETDEV_FRAG_PAGE_MAX_ORDER to zero for Xen (following
> your suggested patch, Ian), and with a different NIC (e1000e
> driven) I'm seeing similar packet loss/corruption on transmits,
> and only if running a debug hypervisor (in a non-debug one,
> MFNs are largely contiguous, so this issue should be observable
> there only very rarely).

I think Konrad is still looking into the underlying swiotlb issue.

If you want to go with the workaround then there is another order>0 to
frob in net/core/sock.c
#define SKB_FRAG_PAGE_ORDER     get_order(32768)
which might help.

Dave Miller unequivocally rejected this approach so I haven't been
pursuing it any further. Perhaps once the swiotlb fix is made it will be
possible to make an argument for it based on actual performance numbers.

Ian.


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