From owner-freebsd-xen@freebsd.org  Wed Jan 25 11:51:05 2017
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Date: Wed, 25 Jan 2017 11:50:51 +0000
From: =?iso-8859-1?Q?=22Roger_Pau_Monn=E9=22?= <roger.pau@citrix.com>
To: Alexander Nusov <alexander.nusov@nfvexpress.com>
CC: Akshay Jaggi <jaggi@freebsd.org>, FreeBSD-Xen <freebsd-xen@freebsd.org>
Subject: Re: Xen on FreeBSD 11: Cannot boot from QCOW2 properly (waiting for
 XENBUS, xenbusb_nop_confighook_cb)
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On Wed, Jan 25, 2017 at 02:20:55PM +0300, Alexander Nusov wrote:
> Got it, thanks.
> 
> 
> 
> I found a workaround to avoid XENBUS delay in Linux DomUs by adding xen_platform_pci = 0 to the configuration file.
> 
> So FreeBSD 11.0 Dom0 can boot Linux guests and Windows Server (FreeBSD DomU still stuck on xenbusb_nop_confighook_cb) from QCOW2 images (also works with QCOW2 overlay images)
> 

On FreeBSD you can boot with QCOW2 by not setting "xen_platform_pci = 0" on the
guest config file and adding "hw.xen.disable_pv_disks=1" to the
/boot/loader.conf file of the guest. This will prevent FreeBSD from using the
PV disk, but you will still get the PV network interfaces.

> 
> Can you tell me please what is the disadvantage of not using /dev/xen/vbd devices and drivers in guests? like slow I/O? 

Slow IO only.

> May it lead to data corruption?

No.