Unmapped I/O

[Poul-Henning Kamp]

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In message <20121219172320.GW71906 at kib.kiev.ua>, Konstantin Belousov writes:

>Still, the i386 cannot have much benefit from the unmapped buffers,
>just because thre is no facilities similar to the direct map for amd64.
>i386 must use transient mapping even for unmapped buffers to copy
>the data to the usermode.

Wrong, a Adaptec 1542 could DMA directly into or out of any spot
of memory and that could have been mapped in userland but not in
kernel.

>Also, as I understand the history, VMIO buffers, or unified page/buffer
>cache, only appeared in the FreeBSD.

Correct, but truth to be told, they have probably delayed our
implementation of unmapped buffers by about 10 years...

I don't blame John & David however, making that full leap in
one go would have required the mythical HeldenProgrammer, there
were a lot of cruft we had to get out of the way first.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.