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Re: one remaining mystery about the FreeBSD domU failure on NetBSD XEN3_DOM0

At Wed, 14 Apr 2021 19:53:47 +0200, Jaromír Doleček <> wrote:
Subject: Re: one remaining mystery about the FreeBSD domU failure on NetBSD XEN3_DOM0
> You can test if this is the problem by disabling the feature in
> negotiation in NetBSD xbdback.c - comment out the code which sets
> feature-max-indirect-segments in xbdback_backend_changed(). With the
> feature disabled, FreeBSD DomU should not use indirect segments.

Ah, yes, thanks!  I should have thought of that.  That's especially
useful since on the client side it's a read-only flag:

	# sysctl -w hw.xbd.xbd_enable_indirect=0
	sysctl: oid 'hw.xbd.xbd_enable_indirect' is a read only tunable
	sysctl: Tunable values are set in /boot/loader.conf

Apparently in the Linux implementation the number of indirect segments
used by a domU can be tuned at boot time, and that appears to be done by
setting a driver option on the guest kernel command line.  When I first
read that it didn't make so much sense to me to be giving this kind of
control to the domU.  Perhaps it would be better to make this a tuneable
in xl.cfg(5) such that it can be tuned on a per-guest basis.  Then
setting it to zero for a given guest would not advertise the feature at

I've some other things to do before I can reboot -- I'll report as soon
as that's done....

					Greg A. Woods <>

Kelowna, BC     +1 250 762-7675           RoboHack <>
Planix, Inc. <>     Avoncote Farms <>

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