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

On Fri, 2012-10-12 at 13:10 +0100, Konrad Rzeszutek Wilk wrote:
> On Fri, Oct 12, 2012 at 07:59:49AM -0400, Konrad Rzeszutek Wilk wrote:
> > On Fri, Oct 12, 2012 at 11:28:08AM +0100, Ian Campbell wrote:
> > > 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.
> > 
> > Ah, that would explain why one of my machines suddenly started
> > developing checksum errors (and had a tg3 card). I hadn't gotten
> > deep into it.
> > > 
> > > 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.
> > 
> > So this should also be easily reproduced on barmetal with 'iommu=soft' then.
> > > 
> > > 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?
> > 
> > The assumption is that it is just a page. I am surprsed that the other
> > IOMMUs aren't hitting this as well - ah, that is b/c they do handle
> > a virtual address of more than one PAGE_SIZE..
> So.. the GART one (AMD poor man IOTLB - was used for AGP card
> translation, but can still be used as an IOMMU - and is still present on
> some AMD machines), looks to suffer the same problem.
> But perhaps not - can you explain to me if a compound page
> is virtually contingous? One of the things the GART does for
> pci_map_single is call page_to_phys(p), feeds the CPU physical address
> (and size) into the GART engine to setup the mapping.
> If compound pages are virtually (and physically on barmetal) contingous
> - this ought to work. But if they are not, then this should also break on
> AMD machines with tg3 and a AMD GART enabled.

AFAIK compound pages are always physically contiguous. i.e. given a
"struct page *page" which is the head of a compound page you can do
"page++" to walk through its constituent frames.

I'm not sure about virtually contiguous. Obviously if they are in lowmem
then the 1-1 map combined with the fact that they are physically
contiguous makes them virtually contiguous too. I'm not sure what
happens if they are highmem -- since kmap (or whatever) would need to do
some extra work in this case. I've not looked but I don't recall
noticing this in the past...


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