From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <62c995f482c487bfe8af18a0af1d3ba9@quanstro.net>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] Re: 9P, procedure calls, kernel, confusion. :(
From: erik quanstrom <quanstro@quanstro.net>
Date: Tue,  5 Feb 2008 09:22:57 -0500
In-Reply-To: <0B8E2530-036F-406C-88D7-02624436ABD1@kix.in>
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one thing of note, linux vfs implements a dcache.  this connects the
virtual memory system to the filesystem.  (but oddly in linux network
buffers are handled seperately.) there is no client-side caching in
the plan 9 kernel.

there is a notable exception.  there is an executable cache.

>> One more point, I googled a lot on "kernel resident file systems and
>> non kernel resident file systems", but I could not find a single
>> useful link. It would be great if you could specify the difference
>> between the two. I wish that eases up the situation a bit.

since the devtab[] functions map 1:1 with 9p, all the mount driver
needs to do for calls outside the kernel is to marshal/demarshal
9p messages.

it's important to remember that most in-kernel file servers could easily
exist outside the kernel.  the entire ip stack can be implemented from
user space.  (and it has been in the past.)

> "Kernel resident filesystem" in this context simply means a filesystem
> which was created for use by the kernel; this may or may not be
> visible to user-space applications - I'm not too sure.

every element of mounttab[] has an associated device letter.  to
mount the device, one does (typically)
	bind -a '#'^$letter /dev

for example, to bind a second ip stack on /net.alt,
	bind -a '#I1' /net.alt

> To sum up, you
> use the 9 primitive operations provided by each 'Dev' when you work
> with kernel-resident filesystems, while all other filesystems are
> dealt with using regular 9P.

all devices are accessed through devtab.  it may be that that entry
is the mount driver.  the mount driver turns devtab[]->fn into
the corresponding 9p message.  (and vice versa.)

- erik