Re: [PATCH 0/7] x86: Address Space Isolation, part 1: per-domain area mapping rework

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.08.2026 13:42, George Dunlap wrote:
> On Mon, Aug 24, 2026 at 10:02 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> While these percentiles in particular of course look very tiny, they are
>> applicable only on systems having no meaningful gaps in the physical
>> address map. And even more generally I find all of these calculations
>> only partly convincing, not the least because you start out from numbers
>> which look pretty contrived when comparing to actual systems which would
>> run the new code. (Using more realistic real-system values may end up
>> going in favor of what you want to convey, or it may not.)
> 
> To be honest, I'm inclined to think that they're not very convincing
> because you don't actually have an idea what the problem is.  You
> didn't specify what you were worried about, so I tried to guess a
> scenario that I considered 95th-percentile worse case.  I don't know
> what kinds of sparse memory layout machines you have in mind -- are
> they written down anywhere, so that contributors can read and
> understand what they need to consider *before* implementing?  Even now
> you haven't even said what about my scenario you consider unrealistic,
> much less told me parameters you think are more realistic.

What I specifically considered unrealistic is that you use huge pCPU and
vCPU counts. Yes, you're trying to do a worst case estimate, yet at the
same time you're assuming huge amounts of memory to be available (which
doesn't represent a "worst case").

As to sparse layouts - ones which have led to the two forms of PDX
compression are well known (I think). The need for more recent (offset)
form is a good example of what could go wrong here: New machines can
always come with new layouts, potentially requiring new compressions
approaches. So what I'm concerned about is effectively _any_ sparse
layout that we may encounter without having a suitable PDX compression
method readily available.

> I don't even know exactly what failure mode you're worried about.  Two
> kinds of potential failures I know about:
>  - Performance impacted because pages can't be NUMA-local
>  - Toolstack operations (including domain creation) fail because
> xenheap has been exhausted.

One thing I can't help thinking you keep overlooking throughout your
reply: xenheap and domheap aren't separate. There being only a
relatively small part of it needed for the worst case estimate you did
means nothing as to exhausting the xenheap in practice: Almost the
entirety of it (with the DMA reserve being somewhat protected) can be
used to build domains. Once in that state, allocations would fail no
matter that large swathes of domheap might (have become) available
(again).

That said, with what you indicated at the very bottom of your reply,
it looks like this part of the discussion has become largely moot.

Jan