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

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Message-ID: <1361506273.2801.327.camel@bling.home>
Date:	Thu, 21 Feb 2013 21:11:13 -0700
From:	Alex Williamson <alex.williamson@...hat.com>
To:	David Gibson <david@...son.dropbear.id.au>
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 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.  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.
Thanks,

Alex

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