physio and vmapbuf

Julian Elischer julian at
Wed Mar 10 18:07:29 UTC 2010

son goku wrote:
> Hi hackers,
> I have some experience with other UNIX kernels but none with FreeBSD.
> FreeBSD interests me for a potential project I might be involved in , and
> therefore I started researching it.
> Browsing through the I/O layer code, I stumbled upon something that looked
> strange, and perhaps you guys can help me with it.
> Mainly, when reading the physio path, when the I/O buffer is from user-mode,
> physio calls vmapbuf. The comments says that vmapbuf actually maps the
> user-buffer into kernel address space. From my experience, such mapping is
> needed only for very old devices that use PIO.

yes, though to some of us that is relatively recent, and in fact we 
still need PIO mode for quite a few devices in the embedded space.
(I have a soekris machine with a flash card that I need to use PIO
mode to reach).

We have been saying for some time (me personally for about 18 years)
that we need to handle unmapped IO better. I don't believe that it has
been done yet (*), but you certainly sound like the right person
to take on that project :-)

(*) I last looked at physio a few years ago so if someone snuck in
unmapped IO in the mean-while, please excuse me.

> Newer device just use DMA for
> data copy, and therefore don't need the mapping, instead they just need to
> pin the memory. Checking out vmapbuf only confused me further, I couldn't
> find where such kernel mapping was taken place, nevertheless I did saw that
> the buf->b_data was updated according to buf->b_saveaddr in some weird
> offset calculation.
> Adding to this confusion, there is a buf field called b_kvabase which
> supposedly stores the kernel virtual address which wasn't touched at all!!
> Any help with understanding the flow and the usage of those fields/variables
> will be greatly appreciated!!
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