linux-kernel - Re: [Intel IOMMU 00/10] Intel IOMMU support, take #2

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Date:	Tue, 26 Jun 2007 07:13:24 -0400
From:	Muli Ben-Yehuda <muli@...ibm.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Keshavamurthy, Anil S" <anil.s.keshavamurthy@...el.com>,
	linux-kernel@...r.kernel.org, gregkh@...e.de,
	suresh.b.siddha@...el.com, arjan@...ux.intel.com,
	ashok.raj@...el.com, davem@...emloft.net, clameter@....com
Subject: Re: [Intel IOMMU 00/10] Intel IOMMU support, take #2

On Tue, Jun 26, 2007 at 09:12:45AM +0200, Andi Kleen wrote:

> There are some potential performance benefits too:
> - When you have a device that cannot address the complete address range
> an IOMMU can remap its memory instead of bounce buffering. Remapping
> is likely cheaper than copying. 

But those devices aren't likely to be found on modern systems.

> - The IOMMU can merge sg lists into a single virtual block. This could
> potentially speed up SG IO when the device is slow walking SG lists.
> [I long ago benchmarked 5% on some block benchmark with an old
> MPT Fusion; but it probably depends a lot on the HBA]

But most devices are SG-capable.

> And you get better driver debugging because unexpected memory
> accesses from the devices will cause an trapable event.

That and direct-access for KVM the big ones, IMHO, and definitely
justify merging.

> > Does it slow anything down?
> 
> It adds more overhead to each IO so yes.

How much? we have numbers (to be presented at OLS later this week)
that show that on bare-metal an IOMMU can cost as much as 15%-30% more
CPU utilization for an IO intensive workload (netperf). It will be
interesting to see comparable numbers for VT-d.

Cheers,
Muli
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