Re: [PATCH 1/4] xen: Introduce non-broken hypercalls for the p2m pool size

[George Dunlap <dunlapg@xxxxxxxxx>]

On Thu, Oct 27, 2022 at 8:12 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:

On 26.10.2022 21:22, Andrew Cooper wrote:
> On 26/10/2022 14:42, Jan Beulich wrote:

 

> paging isn't a great name.  While it's what we call the infrastructure
> in x86, it has nothing to do with paging things out to disk (the thing
> everyone associates the name with), nor the xenpaging infrastructure
> (Xen's version of what OS paging supposedly means).

Okay, "paging" can be somewhat misleading. But "p2m" also doesn't fit
the use(s) on x86. Yet we'd like to use a name clearly better than the
previous (and yet more wrong/misleading) "shadow". I have to admit that
I can't think of any other sensible name, and among the ones discussed
I still think "paging" is the one coming closest despite the
generally different meaning of the word elsewhere.

Inside the world of operating systems / hypervisors, "paging" has always meant "things related to a pagetable"; this includes "paging out to disk".  In fact, the latter already has a perfectly good name -- "swap" (e.g., swap file, swappiness, hypervisor swap).

Grep for "paging" inside of Xen.  We have the paging lock, paging modes, nested paging, and so on.  There's absolutely no reason to start thinking of "paging" as exclusively meaning "hypervisor swap".

 

[ A bunch of stuff about using bytes as a unit size]

> This is going to be a reoccurring theme through fixing the ABIs.  Its
> one of a several areas where there is objectively one right answer, both
> in terms of ease of use, and compatibility to future circumstances.

Well, I wouldn't say using whatever base granularity as a unit is
"objectively" less right.

Personally I don't think bytes or pages either have a particular advantage:

* Using bytes

 - Advantage: Can always use the same number regardless of the underlying page size

 - Disadvantage: "Trap" where if you forget to check the page size, you might accidentally pass an invalid input.  Or to put it differently, most "reasonable-looking" numbers are actually invalid (since most numbers aren't page-aligned)/

* Using pages

 - Advantage: No need to check page alignment in HV, no accidentally invalid input

 - Disadvantage: Caller must check page size and do a shift on every call

What would personally tip me one way or the other is consistency with other hypercalls.  If most of our hypercalls (or even most of our MM hypercalls) use bytes, then I'd lean towards bytes.  Whereas if most of our hypercalls use pages, I'd lean towards pages.

 -George