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

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Date:	Thu, 13 Dec 2007 21:59:50 +0100
From:	Joachim Fenkes <FENKES@...ibm.com>
To:	"Caitlin Bestler" <caitlin.bestler@...erion.com>
Cc:	"Arnd Bergmann" <arnd@...db.de>, caitlin.bestler@...il.com,
	OF-General <general@...ts.openfabrics.org>,
	LKML <linux-kernel@...r.kernel.org>, linuxppc-dev@...abs.org,
	"Or Gerlitz" <ogerlitz@...taire.com>,
	"Roland Dreier" <rdreier@...co.com>,
	Stefan Roscher <stefan.roscher@...ibm.com>
Subject: Re: [ofa-general] Re: [ewg] Re: [PATCH] IB/ehca: Serialize HCA-related
 hCalls on POWER5

caitlin.bestler@...il.com wrote on 13.12.2007 20:22:49:

> On Dec 13, 2007 12:30 AM, Or Gerlitz <ogerlitz@...taire.com> wrote:
> > The current implementation of the open iscsi initiator makes sure to
> > issue commands in thread (sleepable) context, see iscsi_xmitworker and
> > references to it in drivers/scsi/libiscsi.c , so this keeps ehca users
> > safe for the time being.

> I agree, *some* form of FMR support is important for iSER (and probably
> for NFS over RDMA as well). Rather than adding a crippled NO FMR
> mode it would make more sense to add support for FMR Work Requests.
> I'm not certain what, if any, impact that would have on the Power5 
problem,
> but that's certainly a cleaner path for iWARP.

Well, FMR WRs wouldn't change the eHCA issue -- the driver would have to 
make an hCall in any case, and the architecture says that the hCalls used 
in this scenario might return H_LONG_BUSY, causing the driver to sleep. No 
way around that. Because of this, eHCA's FMRs are actually standard MRs 
with a different API.

If, as Or said, the iSCSI initiator issues commands in sleepable context 
anyway, nothing would be lost by using standard MRs as a fallback solution 
if FMRs aren't available, would it?

J.

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