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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Tue, 28 Nov 2023 16:30:27 -0800
Message-ID: <CAJD7tkbxhK7XFcf7h+XE2poNuOsFBQFrxZyeFr=9DoEG_acssA@mail.gmail.com>
Subject: Re: [PATCH 00/16] IOMMU memory observability
To:     Jason Gunthorpe <jgg@ziepe.ca>
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On Tue, Nov 28, 2023 at 4:28=E2=80=AFPM Jason Gunthorpe <jgg@ziepe.ca> wrot=
e:
>
> On Tue, Nov 28, 2023 at 04:25:03PM -0800, Yosry Ahmed wrote:
>
> > > > Right, but as I mention above, if userspace starts depending on thi=
s
> > > > equation, we won't be able to add any more classes of "secondary" p=
age
> > > > tables to SecPageTables. I'd like to avoid that if possible. We can=
 do
> > > > the subtraction in the kernel.
> > >
> > > What Sean had suggested was that SecPageTables was always intended to
> > > account all the non-primary mmu memory used by page tables. If this i=
s
> > > the case we shouldn't be trying to break it apart into finer
> > > counters. These are big picture counters, not detailed allocation by
> > > owner counters.
> >
> > Right, I agree with that, but if SecPageTables includes page tables
> > from multiple sources, and it is observed to be suspiciously high, the
> > logical next step is to try to find the culprit, right?
>
> You can make that case already, if it is high wouldn't you want to
> find the exact VMM process that was making it high?
>
> It is a sign of fire, not a detailed debug tool.

Fair enough. We can always add separate counters later if needed,
potentially under KVM stats to get more fine-grained details as you
mentioned.

I am only worried about users subtracting the iommu-only counter to
get a KVM counter. We should at least document that  SecPageTables may
be expanded to include other sources later to avoid that.

>
> Jason