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

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Message-ID: <alpine.DEB.2.02.1401031659230.8667@kaball.uk.xensource.com>
Date:	Fri, 3 Jan 2014 17:20:54 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad@...nok.org>
CC:	David Vrabel <david.vrabel@...rix.com>,
	<stefano.stabellini@...citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.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, 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:

/* Delay grant-table initialization in the PV on HVM case */
if (xen_hvm_domain())
	return 0;

at the beginning of __gnttab_init.
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