linux-kernel - RE: [ofa-general] Re: Demand paging for memory regions

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Message-ID: <8A71B368A89016469F72CD08050AD334026D5C23@maui.asicdesigners.com>
Date:	Tue, 12 Feb 2008 15:14:23 -0800
From:	"Felix Marti" <felix@...lsio.com>
To:	"Roland Dreier" <rdreier@...co.com>,
	"Christoph Lameter" <clameter@....com>
Cc:	"Rik van Riel" <riel@...hat.com>, <steiner@....com>,
	"Andrea Arcangeli" <andrea@...ranet.com>, <a.p.zijlstra@...llo.nl>,
	<izike@...ranet.com>, <linux-kernel@...r.kernel.org>,
	<avi@...ranet.com>, <linux-mm@...ck.org>,
	<daniel.blueman@...drics.com>, "Robin Holt" <holt@....com>,
	<general@...ts.openfabrics.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	<kvm-devel@...ts.sourceforge.net>
Subject: RE: [ofa-general] Re: Demand paging for memory regions

> -----Original Message-----
> From: general-bounces@...ts.openfabrics.org [mailto:general-
> bounces@...ts.openfabrics.org] On Behalf Of Roland Dreier
> Sent: Tuesday, February 12, 2008 2:42 PM
> To: Christoph Lameter
> Cc: Rik van Riel; steiner@....com; Andrea Arcangeli;
> a.p.zijlstra@...llo.nl; izike@...ranet.com; linux-
> kernel@...r.kernel.org; avi@...ranet.com; linux-mm@...ck.org;
> daniel.blueman@...drics.com; Robin Holt;
general@...ts.openfabrics.org;
> Andrew Morton; kvm-devel@...ts.sourceforge.net
> Subject: Re: [ofa-general] Re: Demand paging for memory regions
> 
>  > > Chelsio's T3 HW doesn't support this.
> 
>  > Not so far I guess but it could be equipped with these features
> right?
> 
> I don't know anything about the T3 internals, but it's not clear that
> you could do this without a new chip design in general.  Lot's of RDMA
> devices were designed expecting that when a packet arrives, the HW can
> look up the bus address for a given memory region/offset and place the
> packet immediately.  It seems like a major change to be able to
> generate a "page fault" interrupt when a page isn't present, or even
> just wait to scatter some data until the host finishes updating page
> tables when the HW needs the translation.

That is correct, not a change we can make for T3. We could, in theory,
deal with changing mappings though. The change would need to be
synchronized though: the VM would need to tell us which mapping were
about to change and the driver would then need to disable DMA to/from
it, do the change and resume DMA.

> 
>  - R.
> 
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