linux-kernel - Re: [PATCH 2/2] iommu/vt-d: Report superpage support in sysfs

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Date:	Mon, 03 Aug 2015 09:09:00 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	iommu@...ts.linux-foundation.org, dwmw2@...radead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iommu/vt-d: Report superpage support in sysfs

On Mon, 2015-08-03 at 16:30 +0200, Joerg Roedel wrote:
> On Tue, Jul 14, 2015 at 03:25:04PM -0600, Alex Williamson wrote:
> > We already have the VT-d capability register printed raw, but it
> > typically involves a trip to the code or the spec to figure out
> > whether superpages are supported.  Make this easier with "2M_pages"
> > and "1G_pages" sysfs entries that clearly report Y/N.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > ---
> >  drivers/iommu/intel-iommu.c |   22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> 
> Can we make this a generic entry for all iommu types and export a
> pagesize bitmap instead?

Are you suggesting a bitmap in order to have a consistent interface,
independently implemented by each IOMMU driver, or are you suggesting a
common interface implemented by iommu_ops.pgsize_bitmap?  The latter is
pretty well broken already.  It can't represent different IOMMU hardware
units having different capabilities, and it's broken by drivers like
intel-iommu that don't expose native hardware page sizes, but anything
that can be broken down into native hardware page size, so ~(4k-1).

Programatically, a bitmap is a concise way to expose multiple page
sizes, but for human consumption, it's not an ideal solution.  Thanks,

Alex

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