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

[Bjorn Helgaas]

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