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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 27 August 2019 08:49
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: 'JulienGrall' <julien.grall@xxxxxxx>; 'Alexandru Isaila' 
> <aisaila@xxxxxxxxxxxxxxx>; 'Petre
> Pircalabu' <ppircalabu@xxxxxxxxxxxxxxx>; 'Razvan Cojocaru' 
> <rcojocaru@xxxxxxxxxxxxxxx>; Andrew Cooper
> <Andrew.Cooper3@xxxxxxxxxx>; George Dunlap <George.Dunlap@xxxxxxxxxx>; Ian 
> Jackson
> <Ian.Jackson@xxxxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>; 
> 'VolodymyrBabchuk'
> <Volodymyr_Babchuk@xxxxxxxx>; 'Stefano Stabellini' <sstabellini@xxxxxxxxxx>; 
> 'xen-
> devel@xxxxxxxxxxxxxxxxxxxx' <xen-devel@xxxxxxxxxxxxxxxxxxxx>; 'Konrad 
> Rzeszutek Wilk'
> <konrad.wilk@xxxxxxxxxx>; 'Tamas K Lengyel' <tamas@xxxxxxxxxxxxx>; Tim 
> (Xen.org) <tim@xxxxxxx>; 'Wei
> Liu' <wl@xxxxxxx>
> Subject: Re: [Xen-devel] [PATCH 3/6] remove late (on-demand) construction of 
> IOMMU page tables
> 
> 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).

TBH I've seen sufficient numbers of domain create failures when using 
auto-ballooning that I stopped using it some time ago (besides the fact that 
it's slow). If you're happy with the simplistic 
double-the-page-table-reservation calculation then I can add that but IMO it 
would be better to add another patch to just remove auto-ballooning.

 Paul

> 
> Jan
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