Re: [linux-pm] [PATCH 0/3] xen: Use PM/Hibernate events for save/restore/chkpt

From: Ian Campbell <Ian.Campbell@eu.citrix.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
	Stabellini <Stefano.Stabellini@eu.citrix.com>Stefano,
	Pavel Machek <pavel@ucw.cz>,
	Shriram Rajagopalan <rshriram@cs.ubc.ca>,
	"linux-pm@lists.linux-foundation.org"
	<linux-pm@lists.linux-foundation.org>
Subject: Re: [linux-pm] [PATCH 0/3] xen: Use PM/Hibernate events for save/restore/chkpt
Date: Mon, 21 Feb 2011 17:17:30 +0000	[thread overview]
Message-ID: <1298308650.16356.8102.camel@zakaz.uk.xensource.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1102211135170.15325-100000@netrider.rowland.org>

On Mon, 2011-02-21 at 16:40 +0000, Alan Stern wrote:
> On Mon, 21 Feb 2011, Ian Campbell wrote:
> 
> > On Sun, 2011-02-20 at 07:49 +0000, Pavel Machek wrote:
> > > On Sat 2011-02-19 15:12:35, Shriram Rajagopalan wrote:
> > > > The current implementation of xen guest save/restore/checkpoint functionality
> > > > uses PM_SUSPEND and PM_RESUME events. This is not optimal when taking
> > > > checkpoints of a virtual machine (where the suspend hypercall returns
> > > > non-zero, requiring the devices and xenbus to just pickup from where they left
> > > > off instead of a complete teardown/reconnect to backend). 
> > > > 
> > > > The following set of patches modify this implementation to use Hibernate style
> > > > control flow (freeze/restore for save/restore and freeze/thaw for checkpoint,
> > > > which is merely a cancelled save akin to failed swsusp() ).
> > > > 
> > > > These patches are against Ian Campbell's PVHVM tree at
> > > > git://xenbits.xen.org/people/ianc/linux-2.6.git for-stefano/pvhvm
> > > > 
> > > > at commit 8a8d1bc753c4e2dda5f2890292d60c67d6ebb573
> > > > kernel version: 2.6.38-rc4
> > > 
> > > Series looks ok to me...
> > 
> > Thanks Pavel, may we take that as an Acked-by?
> > 
> > For my part the Xen side is:
> > Acked-by: Ian Campbell <ian.campbell@citrix.com>
> 
> There's one part of this which could be troublesome.  The new code 
> generates FREEZE, THAW, and RESTORE events even in kernels where 
> CONFIG_HIBERNATION isn't set.  In such kernels, drivers are not 
> obliged to handle these events correctly.

The dependencies on CONFIG_HIBERNATION which I can see appear to be more
often at the bus level (e.g. in drivers/acpi drivers/pci/pci-driver.c
etc) is that right?

For a PV guest only the Xen PV drivers really matter.

But for a PVHVM guest you are right since there are the emulated "PC"
devices though which could be problematic. There's nothing especially
thrilling in that set of devices although I don't think that invalidates
your point.

> Shouldn't the CONFIG_XEN_SAVE_RESTORE option select CONFIG_HIBERNATION?
> In which case the #ifdef lines in pm_op() wouldn't need to be changed.

I think selecting user-visible symbols is generally frowned upon.

But apart from that I was concerned that tying the Xen functionality
into the hibernation option was a bit odd/artificial. Perhaps it's the
only solution though.

Ian.