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

[Muli Ben-Yehuda <muli@il.ibm.com>]

On Tue, Jun 26, 2007 at 05:56:49PM +0200, Andi Kleen wrote:

> > > - 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.
> 
> Your point being?

That the fact that an IOMMU can do SG for non-SG-capble cards is not
interesting from a "reason for inclusion" POV.

> > 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.
> 
> That is something that needs more work.

Yup. I'm working on it (mostly in the context of Calgary) but also
looking at improvements to the DMA-API interface and usage.

> We should probably have a switch to use the IOMMU only for specific
> devices (e.g. for the KVM case) r only when remapping is
> needed.

Calgary already does this internally (via calgary=disable=<BUSNUM>)
but that's pretty ugly. It would be better to do it in a generic
fashion when deciding which dma_ops to call (i.e., a dma_ops per bus
or device).

> Also the user interface for X server case needs more work.

Is anyone working on it?

Cheers,
Muli