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

From: Paul Durrant <Paul.Durrant@citrix.com>
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
Date: Mon, 12 Aug 2019 16:26:25 +0000	[thread overview]
Message-ID: <b7ed5929dd864aefa93bb8572a7b5193@AMSPEX02CL03.citrite.net> (raw)
In-Reply-To: <0134b70d04b942b4b3ce65fd2340ff0c@AMSPEX02CL03.citrite.net>

> -----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?

  Paul
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