linux-kernel - Re: [PATCHv6 5/7] x86/pci: Initial commit for new VMD device driver

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Date:	Thu, 17 Dec 2015 18:25:57 +0000
From:	Keith Busch <keith.busch@...el.com>
To:	Bjorn Helgaas <helgaas@...nel.org>
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>, Martin Mares <mj@....cz>,
	Jon Derrick <jonathan.derrick@...el.com>
Subject: Re: [PATCHv6 5/7] x86/pci: Initial commit for new VMD device driver

On Thu, Dec 17, 2015 at 12:14:48PM -0600, Bjorn Helgaas wrote:
> On Mon, Dec 07, 2015 at 02:32:27PM -0700, Keith Busch wrote:
> > +/*
> > + * VMD h/w converts posted config writes to non-posted. The read-back in this
> > + * function forces the completion so it returns only after the config space was
> > + * written, as expected.
> 
> This comment sounds backwards:
> 
>   posted writes don't wait for completion
>   non-posted writes do wait for completion
> 
> If the hardware converts to non-posted writes, you shouldn't need a
> read-back.  It seems like you would need the read-back if the hardware
> converted non-posted to posted.

Oops, the comment has it backwards. Non-posted config writes become
posted memory write requests with this h/w.
 
> Seems like it might be nice to have something in dmesg that would connect
> this PCI device to the new PCI domain.  It's a new, unusual topology and a
> hint might help everybody understand what's going on.

Sounds good, will add.

In addition that, I'd like to mention this patch links a new domain's
root bus kobject under the VMD end-point's so the sysfs hierarchy captures
this as well.
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