linux-kernel - Re: [Xen-devel] [PATCH 3/7] xen/hvm: Xen PV extension of HVM initialization

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Date:	Tue, 02 Mar 2010 12:17:12 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	Sheng Yang <sheng@...ux.intel.com>,
	Keir Fraser <Keir.Fraser@...citrix.com>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>,
	Ian Pratt <Ian.Pratt@...citrix.com>,
	xen-devel <xen-devel@...ts.xensource.com>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"Yaozu (Eddie) Dong" <eddie.dong@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 3/7] xen/hvm: Xen PV extension of  HVM initialization

On 03/02/2010 01:22 AM, Ian Campbell wrote:
> On Tue, 2010-03-02 at 01:38 +0000, Sheng Yang wrote:
>    
>> A annoy thing in pv drivers is that it would test if the domain type
>> is _NOT_ XEN_NATIVE. So set the domain to XEN_HVM_DOMAIN would result
>> in PV driver initialization then probably panic.
>>      
> What _actually_ panics?
>
> Registration of the frontend devices should be completely harmless
> (apart from a little wasted RAM) unless a xenbus driver manages to come
> up and enumerate the xen bus and cause the ->probe function run.
>
> You should be gating the xenbus startup on the availability of PV
> functionality not the individual driver registrations. This keeps the
> test in a single easy to maintain place.
>
> Compare with pci_register_driver and all of the callers of that function
> -- not a single one of them has an "is_pci_available" test anywhere.
>    

The problem is that it currently assumes xenbus is initialized by the 
time the PV drivers init, because in a PV boot xenbus gets initted very 
early.  We need to change it so that it can cope with drivers being 
initialized and registering with xenbus before it has been initialized.

We have the same problem with plain PV-on-HVM drivers as xenbus only 
comes up as a result of probing the Xen PCI platform device, which may 
be after the PV drivers' init functions have been called.

     J
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