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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.08.2019 11:39, Paul Durrant wrote:
> > -----Original Message-----
> > From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Paul 
> > Durrant
> > Sent: 12 August 2019 17:26
> > To: 'Jan Beulich' <jbeulich@xxxxxxxx>
> > Cc: 'Petre Pircalabu' <ppircalabu@xxxxxxxxxxxxxxx>; 'Stefano Stabellini' 
> > <sstabellini@xxxxxxxxxx>;
> > 'Wei Liu' <wl@xxxxxxx>; 'Razvan Cojocaru' <rcojocaru@xxxxxxxxxxxxxxx>; 'Konrad 
> > Rzeszutek Wilk'
> > <konrad.wilk@xxxxxxxxxx>; Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Tim (Xen.org) 
> > <tim@xxxxxxx>;
> > George Dunlap <George.Dunlap@xxxxxxxxxx>; 'Julien Grall' 
> > <julien.grall@xxxxxxx>; 'Tamas K Lengyel'
> > <tamas@xxxxxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>; 'Alexandru Isaila'
> > <aisaila@xxxxxxxxxxxxxxx>; 'xen-devel@xxxxxxxxxxxxxxxxxxxx' 
> > <xen-devel@xxxxxxxxxxxxxxxxxxxx>;
> > 'VolodymyrBabchuk' <Volodymyr_Babchuk@xxxxxxxx>; Roger Pau Monne 
> > <roger.pau@xxxxxxxxxx>
> > 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.
I'm afraid I disagree: The series extends a pre-existing problem
affecting some guests to all ones (at least by default).

Jan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel