Re: SMMU 2-stage support

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Tue, Apr 14, 2015 at 02:32:26PM +0100, Varun Sethi wrote:
> Hi Will,

Hey Varun,

> > -----Original Message-----
> > From: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org [mailto:iommu-
> > bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org] On Behalf Of Will Deacon
> > Sent: Monday, April 13, 2015 4:11 PM
> > To: Baptiste Reynal
> > Cc: Linux IOMMU
> > Subject: Re: SMMU 2-stage support
> > 
> > On Fri, Apr 03, 2015 at 10:55:02AM +0100, Baptiste Reynal wrote:
> > > We are eventually working on the vSMMU implementation. Relying on the
> > > talk Will Deacon gave at the Linux Plumbers IOMMU Microconference on
> > > October
> > > 2014 (http://linuxplumbersconf.org/2014/ocw/proposals/2019), I tried
> > > the vSMMU initialization.
> > 
> > My position on the vSMMU still hasn't changed:
> > 
> > > > Anyway, until somebody actually wants this feature I've put it on
> > > > ice as it adds a whole bunch of complication to the ARM SMMU driver,
> > > > as well as new user ABI extensions that I don't really want to maintain for
> > fun.
> > 
> > So, whilst it's great that you're looking at the code, I'm not very keen on
> > merging anything until we have people committed to using it. Right now, the
> > only feedback I've had has been going in the para-virt direction and I don't
> > think we should do this "for fun".
> Freescale would be interested in using the vSMMU implementation. We have
> use cases for assigning devices to guest user space. Are you suggesting
> that you are more inclined to using the para virtualized approach?

I can see arguments either way; the vSMMU means that the guest can use the
same SMMU driver as the host but a para-virtualised approach could
theoretically be used across multiple SMMU implementations as well as
potentially being kinder on the TLB.

Will