linux-kernel - Re: [v1 1/2] xen/p2m: Create identity mappings for PFNs beyound E820 and PCI BARs

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Message-Id: <526FE29B02000078000FDB27@nat28.tlf.novell.com>
Date:	Tue, 29 Oct 2013 15:30:19 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>
Cc:	"David Vrabel" <david.vrabel@...rix.com>,
	<Santosh.Jodh@...rix.com>, "Bjorn Helgaas" <bhelgaas@...gle.com>,
	<xhejtman@....muni.cz>,
	"xen-devel" <xen-devel@...ts.xensource.com>,
	<boris.ostrovsky@...cle.com>, <mukesh.rathor@...cle.com>,
	<yuval.shaia@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [v1 1/2] xen/p2m: Create identity mappings for PFNs
 beyound E820 and PCI BARs

>>> On 29.10.13 at 16:11, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
> On Tue, Oct 29, 2013 at 02:55:13PM +0000, Jan Beulich wrote:
>> >>> On 29.10.13 at 15:45, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
>> > On Tue, Oct 29, 2013 at 08:23:30AM +0000, Jan Beulich wrote:
>> >> >>> On 28.10.13 at 17:58, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
>> >> > On Fri, Oct 25, 2013 at 04:08:19PM -0600, Bjorn Helgaas wrote:
>> >> >> If you can look at PCI host bridge apertures instead of BARs, that
>> >> >> would solve both problems.  Reassigning those apertures is
>> >> >> theoretically possible but is not even a gleam in our eyes yet.
>> >> > 
>> >> > <nods> I think I have to have both (BARs and host bridge apertures) as when
>> >> > we do PCI passthrough to a guest - we might do it without a bridge.
>> >> 
>> >> Why? Aren't the host bridge ranges necessarily a superset of the
>> >> individual devices' BARs?
>> > 
>> > Yes. But when you pass in a PCI device to a PV guest you don't pass in the
>> > bridge. Just the PCI device itself.
>> 
>> Right you are. Which means that basing the whole logic on the
>> PCI device BARs is likely wrong anyway, not just because it
>> doesn't account for other MMIO ranges.
> 
> Right, but that is OK. When you pass in a PCI device to a PV guest you
> only care about that specific device driver being able to access its BARs.

Yes, but again - this is not the only possible way of allowing a
guest access to MMIO. See the "iomem" config option.

Jan

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