linux-kernel - Re: [PATCH] IB/ehca: Serialize HCA-related hCalls on POWER5

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Message-ID: <OF9AA24809.F691A5CC-ONC12573AD.006155D6-C12573AD.0062ABE2@de.ibm.com>
Date:	Mon, 10 Dec 2007 18:59:16 +0100
From:	Joachim Fenkes <FENKES@...ibm.com>
To:	Arnd Bergmann <arnd@...db.de>,
	"OF-EWG" <ewg@...ts.openfabrics.org>,
	"OF-General" <general@...ts.openfabrics.org>,
	LKML <linux-kernel@...r.kernel.org>, linuxppc-dev@...abs.org,
	Roland Dreier <rdreier@...co.com>,
	Roland Dreier <rolandd@...co.com>
Cc:	Christoph Raisch <raisch@...ibm.com>,
	Marcus Eder <meder@...ibm.com>,
	Stefan Roscher <stefan.roscher@...ibm.com>
Subject: Re: [PATCH] IB/ehca: Serialize HCA-related hCalls on POWER5

Hi, guys,

> We're taking this to the firmware architects at the moment, but they're 
not 
> very fond of the idea of reporting the absence of bugs through 
capability 
> flags, as this could quickly lead to the exhaustion of flag bits. We'll 
let 
> the discussion stew for a bit, but if we don't get this flag, we'll have 
to 
> resort to the CPU features.

The architects have spoken, and we're getting a capability flag for this. 
I'll repost my patch with new autodetection code that doesn't involve 
checking the processor version.
 
> >  > Regarding the performance problem, have you checked whether 
converting all
> >  > your spin_lock_irqsave to spin_lock/spin_lock_irq improves your 
performance
> >  > on the older machines? Maybe it's already fast enough that way.
> > 
> > It does seem that the only places that the hcall_lock is taken also
> > use msleep, so they must always be in process context.  So you can
> > safely just use spin_lock(), right?
>
> As Arnd said, there are hCalls that will never return H_LONG_BUSY_*, 
such as 
> H_QUERY_PORT and chums, so they will never sleep. The surrounding 
functions, 
> though, are not prepared to be called from interrupt context (GFP_KERNEL 
comes
> to mind), so I agree that a simple spin_lock() will suffice. Thanks, 
Arnd, for
> pointing this out.

As I pointed out in my earlier mail, there's still an issue with 
map_phys_fmr possibly sleeping. Let's keep the irqsave for the time being 
and revisit this part once we find a solution to map_phys_fmr.

Regards,
  Joachim
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