netdev - Re: [PATCH 00/16] IOMMU memory observability

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Date: Tue, 28 Nov 2023 20:54:50 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>, akpm@...ux-foundation.org,
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Subject: Re: [PATCH 00/16] IOMMU memory observability

On Tue, Nov 28, 2023 at 04:30:27PM -0800, Yosry Ahmed wrote:
> On Tue, Nov 28, 2023 at 4:28 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> >
> > On Tue, Nov 28, 2023 at 04:25:03PM -0800, Yosry Ahmed wrote:
> >
> > > > > Right, but as I mention above, if userspace starts depending on this
> > > > > equation, we won't be able to add any more classes of "secondary" page
> > > > > 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 is
> > > > 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.

Well, we just broke it already, anyone thinking it was only kvm
counters is going to be sad now :) As I understand it was already
described to be more general that kvm so probably nothing to do really

Jason