Re: Ordering between PCI config space writes and MMIO reads?

From: Michael S. Tsirkin <mst_at_mellanox.co.il>
Date: 2006-10-26 04:22:52
Quoting r. Matthew Wilcox <matthew@wil.cx>:
> Subject: Re: Ordering between PCI config space writes and MMIO reads?
> 
> On Wed, Oct 25, 2006 at 12:30:22AM -0600, Grant Grundler wrote:
> > Can someone provide a quote of the PCI Local bus spec that allows this?
> > (Or at least a reference to a spec version and section number)
> 
> PCI-PCI bridges are allowed to do it.  If you look in table E-1 of PCI
> 2.3, or table 8-3 of PCI-X 2.0, you'll see that a Posted Memory Write
> can pass a Delayed Write Request (or in PCI-X, a Memory Write can pass a
> Split Write Request).
> 
> So mmiowb() will solve the problem for Altix, but leave everybody else
> vulnerable.  I actually don't see a way of forcing the config write to
> complete before a memory write -- everything is allowed to pass a config
> write, even a config read.  I initially thought "But only a crack monkey
> would implement a system where a config read could pass a config write",
> but the spec explains that:
> 
>   In most PCI-X implementations, Split Requests are managed in separate
>   buffers from Split Completions, so Split Requests naturally pass Split
>   Completions. However, no deadlocks occur if Split Completions block
>   Split Requests.
> 
> So all this code that checks to see if a write had an effect is unsafe.
> I'm a little perturbed by this.  It means the only way to reliably
> distinguish between a write that hasn't taken effect yet and a bit (say,
> MWI) the device hasn't implemented is to do a memory access to the
> device.  Which is hard when you're trying to program the BARs.
> 
> I suppose this hasn't bitten us before in, what, 7 years of PCI-X, so
> it can't be *that* common a thing for bridges to do.  And we would have
> noticed the BAR sizing code going wrong (as it does config write
> followed immediately by config read), so maybe implementations aren't as
> crackful as the PCI spec seems to permit them to be.
> 
> I find it really hard to believe the PCI committee have done something
> this stupid.  There must be another rule somewhere that I'm missing.

I think typically CPUs stall until a non-posted operation completes.
And since config writes are non posted,

pci_config_write_...
write ....

does not *start* the write until config write has completed.
So there's only a single outstanding config operation and that's why
there's never any re-ordering, without any need for flushes.

Your Altix system seems the weird one here in that CPU actually
treats config writes as posted and does not wait for their completion.
I wander whether you can do a bus lock or something and force
waiting till the completion.
This would be much cleaner than trying to fix all drivers.

-- 
MST
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Received on Thu Oct 26 04:22:53 2006

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