linux-kernel - Re: [PATCH] iommu: making IOMMU sysfs nodes API public

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Message-ID: <20130225000322.GN30946@truffula.fritz.box>
Date:	Mon, 25 Feb 2013 11:03:22 +1100
From:	David Gibson <david@...son.dropbear.id.au>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Alexey Kardashevskiy <aik@...abs.ru>,
	Joerg Roedel <joerg.roedel@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu: making IOMMU sysfs nodes API public

On Thu, Feb 21, 2013 at 09:11:13PM -0700, Alex Williamson wrote:
> On Fri, 2013-02-22 at 11:04 +1100, David Gibson wrote:
> > On Tue, Feb 19, 2013 at 01:11:51PM -0700, Alex Williamson wrote:
> > > On Tue, 2013-02-19 at 18:38 +1100, David Gibson wrote:
> > > > On Mon, Feb 18, 2013 at 10:24:00PM -0700, Alex Williamson wrote:
> > > > > On Mon, 2013-02-18 at 17:15 +1100, Alexey Kardashevskiy wrote:
> > [snip]
> > > > >  Adding the window size to sysfs seems more readily convenient,
> > > > > but is it so hard for userspace to open the files and call a couple
> > > > > ioctls to get far enough to call IOMMU_GET_INFO?  I'm unconvinced the
> > > > > clutter in sysfs more than just a quick fix.  Thanks,
> > > > 
> > > > And finally, as Alexey points out, isn't the point here so we know how
> > > > much rlimit to give qemu?  Using ioctls we'd need a special tool just
> > > > to check the dma window sizes, which seems a bit hideous.
> > > 
> > > Is it more hideous that using iommu groups to report a vfio imposed
> > > restriction?  Are a couple open files and a handful of ioctls worse than
> > > code to parse directory entries and the future maintenance of an
> > > unrestricted grab bag of sysfs entries?
> > 
> > The fact that the memory is locked is a vfio restriction, but the
> > actual dma window size is, genuinely, a property of the group.
> 
> A group is an association of devices based on isolation and visibility.
> The dma window happens to be associated with a group on your platform,
> but that's not always the case.

It's certainly an iommu driver specific group property, but a group
property nonetheless.

>  This is why I was hoping something in
> sysfs already reported the dma window so that we could point to it
> rather than creating an interface where it doesn't really belong.

Alas, no.  There's been no reason to expose PEs previously.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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