Re: [PATCH 02/11] xen/page_alloc: Remove `claim` from domain_set_outstanding_pages()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.06.2025 18:42, Roger Pau Monné wrote:
> On Fri, Mar 14, 2025 at 05:24:53PM +0000, Alejandro Vallejo wrote:
>> With a single global count for the claims it's easy to substract
>> domain_tot_pages() from the claim so the number given in the hypercall
>> is the real reservation of the domain. This is the current behaviour.
>>
>> However, a later patch introduces exact-node claims and those interact
>> very poorly with such a scheme. Since accounting domain_tot_pages() in
>> one case but not the other seems strictly worse than not accounting them
>> at all (which is at least consistent), this patch stops substracting
>> tot_pages from the claim and instead checks that claimed memory +
>> allocated memory don't exceed max_mem.
> 
> Hm, while I don't have any specific interest in keeping the current
> behavior, XENMEM_claim_pages is part of the stable ABI (it's not a
> domctl), and hence should be stable.

Is that true? It sits inside a

#if defined(__XEN__) || defined(__XEN_TOOLS__)

which generally de-marks unstable (tools-only) interfaces.

>  Note also the comment above the
> definition of XENMEM_claim_pages how it states the specific behavior
> that you are trying to change (and which should have been adjusted as
> part of this change).

This is the more important part, imo.

> I have no idea why this was made a xenmem rather than a domctl
> hypercall, but if you want to change the semantics I think the only
> option is introducing a new hypercall.

It _is_ a memory operation, and it re-uses one of the interface structs
there. (Yes, none of these would technically have prevented it from
being a domctl.)

Jan