Xen devel - Sep 2010 - Xen 4.0.1, tap vs tap2, blktap2 documentation, and gentoo-xen-kernel problem

Hi,

I''ve been testing Xen 4.0.1 with kernel 2.6.34.5 +
http://gentoo-xen-kernel.googlecode.com/files/xen-patches-2.6.34-4.tar.bz2,
and one of the things that I noticed immediately is there are two
problems with tapdisk and this combination.

First, the syntax "disk = [''tap:tapdisk:aio: ..."  no longer
works.
When tested with HVM domU, it boots, but there was no disk attched
(and /sys/class/blktap2/devices stays empty). Changing it to
tap2:tapdisk:aio works (sort of). I recall some discussion about tap
vs tap2 compatibility on this list earlier, but no final conclusion.
Shouldn''t newer versions be backward compatible? Existing tap:aio and
tap:tapdisk:aio should just work to minimize upgrade efforts.
http://lxr.xensource.com/lxr/source/tools/blktap2/README also still
mention tap:tapdisk, and not tap2:tapdisk.

Second problem, using that combination, blktap2 devices was not
cleanly freed. /sys/class/blktap2/devices and tapdisk devices keep
increasing (tapdev0, tapdev1, etc.). The strange thing is, when using
Xen 4.0.1 hypervisor but with Xen 4.0.0 userland, tap:tapdisk and
blktap2 device cleanup works as expected.

-- 
Fajar

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On Mon, Sep 20, 2010 at 10:02:17AM +0700, Fajar A. Nugraha
wrote:
> Hi,
> 
> I''ve been testing Xen 4.0.1 with kernel 2.6.34.5 +
> http://gentoo-xen-kernel.googlecode.com/files/xen-patches-2.6.34-4.tar.bz2,
> and one of the things that I noticed immediately is there are two
> problems with tapdisk and this combination.
> 
> First, the syntax "disk = [''tap:tapdisk:aio: ..."  no
longer works.
> When tested with HVM domU, it boots, but there was no disk attched
> (and /sys/class/blktap2/devices stays empty). Changing it to
> tap2:tapdisk:aio works (sort of). I recall some discussion about tap
> vs tap2 compatibility on this list earlier, but no final conclusion.
> Shouldn''t newer versions be backward compatible? Existing tap:aio
and
> tap:tapdisk:aio should just work to minimize upgrade efforts.
> http://lxr.xensource.com/lxr/source/tools/blktap2/README also still
> mention tap:tapdisk, and not tap2:tapdisk.
> 
http://wiki.xensource.com/xenwiki/blktap2

The syntax was changed in Xen 4.0.1 .. which is a bit confusing..
> Second problem, using that combination, blktap2 devices was not
> cleanly freed. /sys/class/blktap2/devices and tapdisk devices keep
> increasing (tapdev0, tapdev1, etc.). The strange thing is, when using
> Xen 4.0.1 hypervisor but with Xen 4.0.0 userland, tap:tapdisk and
> blktap2 device cleanup works as expected.
> 
Interesting. Did you try with a pvops dom0 kernel? Does that make a difference?

-- Pasi


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