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

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Date:   Wed, 15 Mar 2017 09:09:15 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andi Kleen <andi@...stfloor.org>, Andi Kleen <ak@...ux.intel.com>,
        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 Wed, Mar 15, 2017 at 11:00:22AM +0100, Thomas Gleixner wrote:
> On Tue, 14 Mar 2017, Bjorn Helgaas wrote:
> > On Tue, Mar 14, 2017 at 07:24:14PM -0700, Andi Kleen wrote:
> > > > 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.
> > > 
> > > That's what my other patch already did. 
> > 
> > Yes, your 1/4 patch does add the "ll_allowed" bit in struct pci_ops.
> > 
> > What I was wondering, but didn't explain very well, was whether
> > instead of setting that bit at run-time in pci_mmcfg_arch_init(), we
> > could set it statically in the pci_ops definition, e.g.,
> > 
> >   static struct pci_ops ecam_ops = {
> >     .lockless = 1,
> >     .read = ecam_read,
> >     .write = ecam_write,
> >   };
> > 
> > I think it would be easier to read if the lockless-ness were declared
> > right next to the accessors that need it (or don't need it).
> > 
> > But it is a little confusing with all the different paths, at least on
> > x86, so maybe it wouldn't be quite that simple.
> 
> The pci_ops in x86 are a complete mess.

That's certainly a pithy summary :)

> pci_root_ops is what is finally handed in to pci_scan_root_bus() as ops
> argument for any bus segment no matter which type it is.
> 
> The locking aspect is interesting as well. The type0/1 functions are having
> their own internal locking. Oh, well.
> 
> What we really want is to differentiate bus segments. That means a PCIe
> segment takes mmconfig ops and a PCI segment the type0/1 ops. That way we
> can do what you suggested above, i.e. marking the ecam/mmconfig ops as
> lockless.

If we were starting from scratch, I think we would probably put the
locking inside the device-specific config accessors at the lowest
level.  Then it would be directly at the place where it's obvious
what's needed, and it would be easy to do no locking, per-host bridge
locking, or system-wide locking.  Right now we have many places that
implicitly depend on pci_lock but there's no direct connection.

We could conceivably migrate to that, but it would be a fair amount of
work.

Bjorn