[Xen-devel] Reversing the DMA handle

[Pete Zaitcev <zaitcev@xxxxxxxxxx>]

Dear Xen developers:

I maintain a component in Linux kernel, called usbmon. It provides for
snooping of USB traffic, analoguously to tcpdump. Due to pequliarities
of USB API in Linux, usbmon has to take a DMA address (coming from
dma_alloc_coherent() typically), find the corresponding PFN and call
pkmap()/memcpy()/pkunmap().

My question is: how to find PFN under Xen if DMA address is known,
on x86_64 and i386?

I see some functions which may be useful (e.g. mfn_to_pfn()), but what
I need the most is some tribal knowledge about what is the proper way
to do it. This includes things like working in both dom0 and domU,
not faulting in machine_to_phys_mapping[] while in hot path, and so on.

Thank you,
-- Pete

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