linux-kernel - Re: [PATCH] xen/xenbus: Add quirk to deal with misconfigured backends.

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Date:	Thu, 22 Mar 2012 10:52:26 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"JBeulich@...ell.com" <JBeulich@...ell.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH] xen/xenbus: Add quirk to deal with misconfigured
 backends.

On Thu, 2012-03-22 at 10:44 +0000, Stefano Stabellini wrote:
> On Thu, 22 Mar 2012, Konrad Rzeszutek Wilk wrote:
> > A rather annoying and common case is when booting a PVonHVM guest
> > and exposing the PV KBD and PV VFB - as both of those do not
> > make any sense. The HVM guest is using the VGA driver and the emulated
> > keyboard for this. So we provide a very basic quirk framework
> > (can be expanded in the future) to not wait for 6 minutes for those devices
> > to initialize - they never wont.
> > 
> > To trigger this, put this in your guest config:
> > 
> > vfb = [ 'vnc=1, vnclisten=0.0.0.0 ,vncunused=1']
> > 
> > instead of this:
> > vnc=1
> > vnclisten="0.0.0.0"
> 
> While I do understand the issue you are trying to solve, it actually
> makes sense to have PV KBD (and PV VFB maybe in the future) in a PVonHVM
> guest. In particular PV KVB is already enabled in upstream QEMU for
> PVonHVM guests because it allows users to have a keyboard and mouse
> without USB emulation, that requires lots of wakes up in QEMU.
> 
> Maybe we could just reduce the timeout in general for all the PV
> devices? After all, why are we waiting 6 minutes? I could understand 6
> seconds, but 6 minutes seem really too much.

This was increased based on empirical evidence, way back (circa
150:09c88868e344 in linux-2.6.18-xen.hg)

It really can happen when starting lots of guests on a heavily loaded
dom0 that you timeout when connecting devices, at which point the guest
fails to boot if it happened to contain the root filesystem.

Maybe a halfway house would be to wait a the longer time for more
critical devices (like disks and nics)?

Ian.

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