linux-kernel - Re: [PATCH 3/4] x86, pci: Add interface to force mmconfig

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Message-ID: <20170314194720.GD26264@bhelgaas-glaptop.roam.corp.google.com>
Date:   Tue, 14 Mar 2017 14:47:20 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andi Kleen <ak@...ux.intel.com>, Andi Kleen <andi@...stfloor.org>,
        bhelgaas@...gle.com, x86@...nel.org, linux-pci@...r.kernel.org,
        eranian@...gle.com, Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] x86, pci: Add interface to force mmconfig

On Tue, Mar 14, 2017 at 06:56:26PM +0100, Thomas Gleixner wrote:
> On Tue, 14 Mar 2017, Andi Kleen wrote:
> > The other option is to simply make it unconditional. That would
> > be even simpler, but it is a bit more risky. Hmm, I suppose may
> > be worth trying to find out what Windows uses. If they get away
> > with MMCONFIG everywhere we should be too.
> 
> From the PCIe spec:
> 
>  The PCI 3.0 compatible Configuration Space can be accessed using either
>  the mechanism defined in the PCI Local Bus Specification or the PCI
>  Express Enhanced Configuration Access Mechanism (ECAM) described later in
>  this section. Accesses made using either access mechanism are
>  equivalent. The PCI Express Extended Configuration Space can only be
>  accessed by using the ECAM.
> 
> The 1.0 spec has a similar wording (s/3.0/2.3).
> 
> Definitely the best way to go.

I agree that it should be fairly safe to do ECAM/MMCONFIG without
locking.  Can we handle the decision part by adding a "lockless" bit
to struct pci_ops?  Old ops don't mention that bit, so it will be
initialized to zero and we'll do locking as today.  ECAM/MMCONFIG ops
can set it and we can skip the locking.

Bjorn