Re: [Xen-devel] [PATCH 2/4] iommu: generalize iommu_inclusive_mapping

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Paul Durrant [mailto:Paul.Durrant@xxxxxxxxxx]
> Sent: Wednesday, August 1, 2018 4:34 PM
> 
> > -----Original Message-----
> > From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On
> Behalf
> > Of Jan Beulich
> > Sent: 01 August 2018 09:21
> > To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> > Cc: Kevin Tian <kevin.tian@xxxxxxxxx>; Stefano Stabellini
> > <sstabellini@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; George Dunlap
> > <George.Dunlap@xxxxxxxxxx>; Andrew Cooper
> > <Andrew.Cooper3@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>;
> Tim
> > (Xen.org) <tim@xxxxxxx>; Julien Grall <julien.grall@xxxxxxx>; xen-devel
> > <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> > Subject: Re: [Xen-devel] [PATCH 2/4] iommu: generalize
> > iommu_inclusive_mapping
> >
> > >>> On 31.07.18 at 17:33, <roger.pau@xxxxxxxxxx> wrote:
> > > On Tue, Jul 31, 2018 at 08:39:22AM -0600, Jan Beulich wrote:
> > >> >>> On 27.07.18 at 17:31, <roger.pau@xxxxxxxxxx> wrote:
> > >> > Introduce a new iommu=inclusive generic option that supersedes
> > >> > iommu_inclusive_mapping. This should be a non-functional change
> on
> > >> > Intel hardware, while AMD hardware will gain the same functionality
> of
> > >> > mapping almost everything below the 4GB boundary.
> > >>
> > >> So first of all - what's the motivation behind this change? So far we
> > >> had no need for hacks line the VT-d side one on AMD. I don't think
> > >> this should be widened without there being indication of a problem
> > >> with non-niche AMD systems.
> > >
> > > OK, I can leave the default on for Intel and off for everything else,
> > > but I will introduce the generic dom0-iommu= option anyway.
> >
> > Hmm, I've always been wishing we'd change to a default of off for
> > VT-d as well - imo we shouldn't by default assume broken firmware.
> > Kevin?
> 
> I suspect that a seriously large number of Xen users will find their systems
> fail to boot if the default is changed. I'm testing on a Dell R730 with (to my
> knowledge) up-to-date firmware. Not exactly a rare system and turning off
> inclusive mapping will cause it to wedge during boot.
> 
>   Paul

Though I agree with Jan in concept, the reality means we have to
keep default on as before. :-)

Thanks
Kevin
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