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

[Alex Williamson <alex.williamson@redhat.com>]

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@redhat.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