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To: Konstantin Belousov <kostikbel@gmail.com>
Subject: Re: Unmapped I/O
In-reply-to: <20121219183600.GX71906@kib.kiev.ua>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
References: <20121219135451.GU71906@kib.kiev.ua> <50D1D2BD.80107@freebsd.org>
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Date: Wed, 19 Dec 2012 18:47:23 +0000
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Cc: mjacob@freebsd.org, freebsd-arch@freebsd.org
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--------
In message <20121219183600.GX71906@kib.kiev.ua>, Konstantin Belousov writes:

>> 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.
>
>And how this can be used while keeping on-disk data coherent with the
>buffer ?[...]

You simply don't care about the kernel buffer (most of the time).

The kernel doesn't need to see the data, all it has to do is move
it from disk to userland.

Of course there are boundary issues and cornercases that
need to be handled, for instance if userland does not issue requests
which are disk-sector-integral, in which case the buffers will be
needed, but for the common case, they will not be necessary.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@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.