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

From: George Dunlap <dunlapg@umich.edu>
To: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <Andrew.Cooper3@citrix.com>,
	Roger Pau Monne <roger.pau@citrix.com>,  Wei Liu <wl@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Julien Grall <julien@xen.org>,
	 Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
	Bertrand Marquis <bertrand.marquis@arm.com>,
	 Henry Wang <Henry.Wang@arm.com>,
	Anthony Perard <anthony.perard@citrix.com>,
	 Xen-devel <xen-devel@lists.xenproject.org>,
	George Dunlap <george.dunlap@citrix.com>
Subject: Re: [PATCH 1/4] xen: Introduce non-broken hypercalls for the p2m pool size
Date: Fri, 28 Oct 2022 16:27:21 +0100	[thread overview]
Message-ID: <CAFLBxZbWkLSMxXAYRGYc9Z3Vvj6bT+m7nvdiZgWRdr+_nF0BfQ@mail.gmail.com> (raw)
In-Reply-To: <8a8bc184-6237-ed24-8d9f-daa3c36df915@suse.com>

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On Thu, Oct 27, 2022 at 8:12 AM Jan Beulich <jbeulich@suse.com> 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

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