linux-kernel - Re: [RFC PATCHv2] x86/pci: Initial commit for new VMD device driver

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Date:	Thu, 8 Oct 2015 08:29:03 -0500
From:	Bjorn Helgaas <helgaas@...nel.org>
To:	Keith Busch <keith.busch@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	linux-pci@...r.kernel.org, Jiang Liu <jiang.liu@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dan Williams <dan.j.williams@...el.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Bryan Veal <bryan.e.veal@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC PATCHv2] x86/pci: Initial commit for new VMD device driver

On Wed, Oct 07, 2015 at 04:04:07PM +0000, Keith Busch wrote:
> On Tue, 6 Oct 2015, Keith Busch wrote:
> >On Tue, 6 Oct 2015, Bjorn Helgaas wrote:
> >>>+	resource_list_for_each_entry(entry, &resources) {
> >>>+		struct resource *source, *resource = entry->res;
> >>>+
> >>>+		if (!i) {
> >>>+			resource->start = 0;
> >>>+			resource->end = (resource_size(
> >>>+					&vmd->dev->resource[0]) >> 20) - 1;
> >>>+			resource->flags = IORESOURCE_BUS | IORESOURCE_PCI_FIXED;
> >>
> >>I thought BAR0 was CFGBAR.  I missed the connection to a bus number
> >>aperture.
> >
> >Right, BAR0 is the CFGBAR and is the device's aperture to access its
> >domain's config space.
> 
> It's a new day, I'll try a new explanation on what this is about. The size
> of the CFGBAR determines how many bus numbers can be reached through the
> device's config space aperture. We are not setting the bus resource to
> BAR0; just determining the bus resource ending based on BAR0's size. We
> expect the bar to be 256M to access config space for 256 buses:
> 
>   8 functions * 32 devices * 256 buses * 4k config space per function = 256M
> 
> If the BAR wasn't provided 256M for any reason, we reduce the number of
> bus resources this domain can provide.

Yep, that makes sense.  A comment here about the fact that BAR0 maps
ECAM space for the new domain might help.

Thanks!

Bjorn
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