linux-kernel - Re: [PATCH] xen/pvhvm: If xen_platform

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Message-ID: <1386155291.17466.19.camel@kazak.uk.xensource.com>
Date:	Wed, 4 Dec 2013 11:08:11 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>,
	<boris.ostrovsky@...cle.com>, <david.vrabel@...rix.com>,
	<leosilva@...ux.vnet.ibm.com>, <ashley@...leylai.com>,
	<peterhuewe@....de>, <mail@...jiv.net>, <tpmdd@...horst.net>,
	<tpmdd@...rix.com>, <dmitry.torokhov@...il.com>,
	<bhelgaas@...gle.com>, <plagnioj@...osoft.com>,
	<tomi.valkeinen@...com>, <tpmdd-devel@...ts.sourceforge.net>,
	<linux-input@...r.kernel.org>, <netdev@...r.kernel.org>,
	<linux-pci@...r.kernel.org>, <linux-fbdev@...r.kernel.org>
Subject: Re: [PATCH] xen/pvhvm: If xen_platform_pci=0 is set don't blow up.

On Wed, 2013-12-04 at 11:05 +0000, Stefano Stabellini wrote:
> On Wed, 4 Dec 2013, Ian Campbell wrote:
> > On Wed, 2013-12-04 at 10:51 +0000, Stefano Stabellini wrote:
> > > On Wed, 4 Dec 2013, Ian Campbell wrote:
> > > > > +bool xen_has_pv_devices(void)
> > > > > +{
> > > > > +	if (!xen_domain())
> > > > > +		return false;
> > > > > +
> > > > > +	if (xen_hvm_domain()) {
> > > > > +		/* User requested no unplug, so no PV drivers. */
> > > > > +		if (xen_emul_unplug & XEN_UNPLUG_NEVER)
> > > > > +			return false;
> > > > 
> > > > I think you need
> > > > 		if (xen_emul_unpug & XEN_UNPLUG_UNNECESSARY)
> > > > 			return true;
> > > > don't you?
> > > 
> > > XEN_UNPLUG_UNNECESSARY was introduced to enable the platform PCI device
> > > even if it didn't respond properly to the unplug protocol.
> > > The corresponding parameter is called "unnecessary" because if you pass
> > > it to the kernel you mean that it is unnecessary to unplug the emulated
> > > devices but you can use the pv devices anyway.
> > > 
> > > So no, we shouldn't check for XEN_UNPLUG_UNNECESSARY here.
> > 
> > Oh, we will eventually fall through to the return true, so it does
> > actually work out OK.
> > 
> > I'd still be in favour of handling each option explicitly, for clarity.
> > Which means checking for XEN_UNPLUG_UNNECESSARY.
> 
> I think is wrong to check for any xen_emul_unpug options in this function.
> The xen_emul_unpug options should be used to set the right value of
> xen_platform_pci_unplug. (See my other reply.)

Whichever one we check we should still be checking explicitly for the
"unnecessary" case, for clarity if nothing else.

TBH I think the split between xen_emul_unplug and
xen_platform_pci_unplug is a bit artificial. There should be one value
which is static to platform-pci-unplug.c and accessor functions should
be provided for other code to use. Open coding all those accesses to
xen_platform_pci_unplug in every driver is just too error prone.

Ian.

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