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====== Piotr Jaroszyński: Usermode debugging under Linux ======
===== Week 7 [ Jul 5 - Jul 11 2010 ] =====

==== drivers in userspace ====

After a lot of reading and trial and error I have managed to come up with a working, but awfully hacky proof of concept:

<code>
# ./pcnet32.linuxlibc 
gPXE initialising devices...



gPXE 1.0.1+ -- Open Source Boot Firmware -- http://etherboot.org
Features: HTTP DNS TFTP
DHCP (net0 52:54:00:12:34:58).... ok
http://root.piotrj.org/files/gpxe/1mb. ok
</code>

=== PIO ===
To make the ''pci_(read|write)_config_*()'' work I picked ''PCIAPI_DIRECT'' in ''config/ioapi.h'' and made all I/O ports available to userspace with ''iopl(3)''.
That also had a side-effect of making rest of the ''PIO'' used by drivers work. The question remains how portable ''PCIAPI_DIRECT'' is.
Answering that requries more reading. Other approaches are possible here as the PCI config interface is exposed in ''/sys/'' and there is even a lib in ''pciutils'' for accessing it.

=== MMIO ===

Haven't looked at that yet. A kernel module might be required to mark the pages mmaped to hardware as non-cacheable and non-prefetchable.

=== DMA ===

And to make DMA work ''core/malloc.c'' uses a mmaped arbitrary (I just tried several ranges until it worked) chunk of ''/dev/mem''. This is ridiculously bad as it can end up corrupting kernel structures, but that's what kvm is for :)

Obviously this has to be changed. Stefan suggested that it might be possible to ''mlock()'' the ''heap'' (so that it's not swapped away) from ''core/malloc.c'' and figure out its physical address via ''/proc/pagemap''.
That doesn't guarantee for it to be physically contiguous, but at least we can verify that.

Doing it properly seems to require a kernel module. MMIO might too and I was supposed to look at UIO anyway, so that's on the radar now.

Trace:

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