Re: [Xen-devel] [PATCH 3/6] remove late (on-demand) construction of IOMMU page tables

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@lists.xenproject.org> On Behalf Of Paul Durrant
> Sent: 12 August 2019 17:26
> To: 'Jan Beulich' <jbeulich@suse.com>
> Cc: 'Petre Pircalabu' <ppircalabu@bitdefender.com>; 'Stefano Stabellini' <sstabellini@kernel.org>;
> 'Wei Liu' <wl@xen.org>; 'Razvan Cojocaru' <rcojocaru@bitdefender.com>; 'Konrad Rzeszutek Wilk'
> <konrad.wilk@oracle.com>; Andrew Cooper <Andrew.Cooper3@citrix.com>; Tim (Xen.org) <tim@xen.org>;
> George Dunlap <George.Dunlap@citrix.com>; 'Julien Grall' <julien.grall@arm.com>; 'Tamas K Lengyel'
> <tamas@tklengyel.com>; Ian Jackson <Ian.Jackson@citrix.com>; 'Alexandru Isaila'
> <aisaila@bitdefender.com>; 'xen-devel@lists.xenproject.org' <xen-devel@lists.xenproject.org>;
> 'VolodymyrBabchuk' <Volodymyr_Babchuk@epam.com>; Roger Pau Monne <roger.pau@citrix.com>
> Subject: Re: [Xen-devel] [PATCH 3/6] remove late (on-demand) construction of IOMMU page tables
> 
> > -----Original Message-----
> [snip]
> > >
> > > On 30.07.2019 15:44, Paul Durrant wrote:
> > > > NOTE: This patch will cause a small amount of extra resource to be used
> > > >        to accommodate IOMMU page tables that may never be used, since the
> > > >        per-domain IOMMU flag enable flag is currently set to the value
> > > >        of the global iommu_enable flag. A subsequent patch will add an
> > > >        option to the toolstack to allow it to be turned off if there is
> > > >        no intention to assign passthrough hardware to the domain.
> > >
> > > In particular if the default of this is going to be "true" (I
> > > didn't look at that patch yet, but the wording above makes me
> > > assume so), in auto-ballooning mode without shared page tables
> > > more memory should imo be ballooned out of Dom0 now. It has
> > > always been a bug that IOMMU page tables weren't accounted for,
> > > but it would become even more prominent then.
> >
> > Ultimately, once the whole series is applied, then nothing much should change for those specifying
> > passthrough h/w in an xl.cfg. The main difference will be that h/w cannot be passed through to a
> > domain that was not originally created with IOMMU pagetables.
> > With patches applied up to this point then, yes, every domain will get IOMMU page tables. I guess
> I'll
> > take a look at the auto-ballooning code and see what needs to be done.
> >
> 
> Ok, I've had a look...
> 
> I could make a rough calculation in libxl_domain_need_memory() based on the domain's max_memkb and an
> assumption of a 4 level translation with 512 PTEs per level, or would prefer such guestimation to be
> overridable using an xl.cfg parameter in a broadly similar way to shadow_memkb?
> 

I think I'm going to say no to this anyway since, as you say, the overhead as never been accounted for and having to make assumptions about the IOMMU table structure is not very attractive. Given that any issue is going to be transient (i.e. until patch #6 is committed) I don't think fixing auto-ballooning ought to be in scope for this series.

  Paul

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