linux-kernel - Re: [Xen-devel] [PATCH v12 15/18] xen/pvh: Piggyback on PVHVM for grant driver (v2)

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Message-ID: <20140103183956.GA29058@phenom.dumpdata.com>
Date:	Fri, 3 Jan 2014 13:39:56 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH v12 15/18] xen/pvh: Piggyback on PVHVM for
 grant driver (v2)

On Fri, Jan 03, 2014 at 06:29:25PM +0000, Stefano Stabellini wrote:
> On Fri, 3 Jan 2014, Konrad Rzeszutek Wilk wrote:
> > On Fri, Jan 03, 2014 at 05:20:54PM +0000, Stefano Stabellini wrote:
> > > On Fri, 3 Jan 2014, Konrad Rzeszutek Wilk wrote:
> > > > On Fri, Jan 03, 2014 at 03:41:51PM +0000, David Vrabel wrote:
> > > > > On 03/01/14 14:44, Konrad Rzeszutek Wilk wrote:
> > > > > > On Fri, Jan 03, 2014 at 11:54:13AM +0000, David Vrabel wrote:
> > > > > >> On 02/01/14 18:50, Konrad Rzeszutek Wilk wrote:
> > > > > >>> On Thu, Jan 02, 2014 at 04:32:03PM +0000, David Vrabel wrote:
> > > > > >>>> On 01/01/14 04:35, Konrad Rzeszutek Wilk wrote:
> > > > > >>>>> @@ -1320,4 +1323,4 @@ static int __gnttab_init(void)
> > > > > >>>>>  	return gnttab_init();
> > > > > >>>>>  }
> > > > > >>>>>  
> > > > > >>>>> -core_initcall(__gnttab_init);
> > > > > >>>>> +core_initcall_sync(__gnttab_init);
> > > > > >>>>
> > > > > >>>> Why has this become _sync?
> > > > > >>>
> > > > > >>> It needs to run _after_ the xen_pvh_gnttab_setup has run (which is
> > > > > >>> at gnttab_init):
> > > > > >>
> > > > > >>
> > > > > >> The use of core_initcall_sync() doesn't imply any ordering to me.  Can't
> > > > > > 
> > > > > > It has a clear ordering property.
> > > > > 
> > > > > This really isn't obvious to me.  Can you point to the docs/code the
> > > > > guarantee this?  I couldn't find it.
> > > > 
> > > > include/linux/init.h
> > > > > 
> > > > > >> you call xen_pvh_gnttab_setup() from within __gnttab_init() ?
> > > > > > 
> > > > > > No. That is due to the fact that __gnttab_init() is in drivers/xen and is
> > > > > > also used by the ARM code.
> > > > > > 
> > > > > > Stefano in his previous review mentioned he would like PVH specific
> > > > > > code in arch/x86:
> > > > > > 
> > > > > > https://lkml.org/lkml/2013/12/18/507
> > > > > 
> > > > > Call it xen_arch_gnttab_setup() and add weak stub for other architectures?
> > > > 
> > > > Stefano, thoughts?
> > > 
> > > I think that you can safely move __gnttab_init to postcore_initcall if
> > > it works correctly for the PV and PVH cases, because HVM and ARM are
> > > unaffected by it.  In fact they don't initialize the grant table via
> > > __gnttab_init at all. See:
> > 
> > The 'xenbus_init' is called in postcore_initcall. I don't actually
> > know if it is OK to call that _before_ gnttab_init is called.
> 
> No, xenbus_init needs to be called after gnttab_init, however the
> alphabetical order would enforce it.
> Not that I would want to rely on it :-)

Exactly. Which is why I came back to the idea of just moving __gnttab_init 
one level down in the '1' runlevel. This way I can guarantee that this
order of operation will be done:

xen_pvh_gnttab_setup
__gnttab_init
xenbus_init

Without anybody coming up with a patch that would randomize the order
of functions called within the runlevels.

I gather you prefer then this approach then?

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