On Wed, 23 Oct 2013, Konrad Rzeszutek Wilk wrote:

On Wed, Oct 23, 2013 at 06:34:13PM +0100, Stefano Stabellini wrote:

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On Thu, Oct 17, 2013 at 06:43:33PM +0100, Stefano Stabellini wrote:

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xen_unmap_single calls xen_bus_to_phys, then passes dev_addr to
is_xen_swiotlb_buffer that calls xen_bus_to_phys again.

It does?

I see it call mfn_to_local_pfn which calls pfn_to_mfn and
get_phys_to_machine (which I presume is going to be a slow
operation on ARM).

Yeah, I am trying to reduce the number of pfn_to_mfn and mfn_to_pfn
translations.
That said, this patch is not needed to have a fully functional
swiotlb-xen on arm and I would OK with postponing it.

OK, lets do that as it requires some hard thinking.

OK.

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I am bit uncomfortable with this - I recall when developing this
that I had hit some odd cases of doing DMA of a page to a guest
and it not always working. It was the case of not detecting whether
the pfn was foreign or not. Sadly I can't recall the details.

I would think that this problem would also be present with ARM?
But perhaps all the new changes in the pfn_to_mfn that had lately
been done have fixed this?

No, because on ARM the guest is autotranslate so the pfn returned by
mfn_to_pfn is always "local".
However thanks to the m2p_override now we should get the "local" pfn
from mfn_to_pfn on x86 too.
I'll test this case though.

Right, so maybe the pfn_to_mfn_local test is useless nowadays?

Yes, I think so. This might be better handled by removing
mfn_to_local_pfn altogether.