From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Campbell <Ian.Campbell@citrix.com>
Date: Wed, 04 Dec 2013 11:08:11 +0000
Subject: Re: [PATCH] xen/pvhvm: If xen_platform_pci=0 is set don't blow up.
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To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, boris.ostrovsky@oracle.com, david.vrabel@citrix.com, leosilva@linux.vnet.ibm.com, ashley@ashleylai.com, peterhuewe@gmx.de, mail@srajiv.net, tpmdd@selhorst.net, tpmdd@sirrix.com, dmitry.torokhov@gmail.com, bhelgaas@google.com, plagnioj@jcrosoft.com, tomi.valkeinen@ti.com, tpmdd-devel@lists.sourceforge.net, linux-input@vger.kernel.org, netdev@vger.kernel.org, linux-pci@vger.kernel.org, linux-fbdev@vger.kernel.org

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.