[Xen-devel] Re: [PATCH] have udev create the device for blktap

[Steven Rostedt <srostedt@xxxxxxxxxx>]

Andrew Warfield wrote:
> I'm wondering about this one a bit, as I'm not so familiar with the
> class stuff.  This patch isn't changing any of the device creation
> code -- it's just adding calls to publish the existence of the
> tap-related device nodes through sysfs so that udev creates them for
> us, right?
Yep.

> Can you explain what the new 'xen' class does -- is it just to keep
> things organized within /sys?  the /dev/xen subdirectory is created
> through the udev rules, and not implicitly through having the class,
> isn't it?
The class seemed the most reasonable way to organize it in sysfs.  The 
directory is still created by the udev rules, but that is still the norm.
> Assuming this is true, I wonder if it's worth broadening this patch
> slightly to also incorporate the event channel driver, and to pull the
> xen  class stuff out into drivers/xen/core/sysfs_class.c or similar.
That can be done.  I was going to do something with the evtchn but since 
it was a misc device, it is automatically registered in the sysfs 
system.  And udev uses that to create it.  Although we should probably 
have the evtchn grab a dynamic minor from the misc system.  And for 
those systems without udev, we could have the daemon read the sysfs 
system to find the minor. (I should probably change the blktapctrl to 
read sysfs instead of /proc/devices).
As noted in the code, I would have put it else where (perhaps it's own 
file), but since blktap was the only user (so far) I kept it there. Hmm, 
I guess I can through that code into its own file.  Would be a small 
file though :)   Would that be the preferred method?
> Also, on the blocktap side, it would be nice (in a later patch) to
> kill the static allocation of the data-path devices and allocate them
> on demand, as you do with the associed sysfs entries.
That would not be a problem.  The tap devices could be stored in a link 
list. The only time you would need to search the list to find the tap 
device, is really on open.  You would also do it to look for a free 
device, but that search is already done.
I'll take a look into that too.


-- Steve


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