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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.11.2022 02:19, Stefano Stabellini wrote:
> On Fri, 28 Oct 2022, George Dunlap wrote:
>> 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.
> 
> 
> Joining the discussion late to try to move things forward.
> 
> Let me premise that I don't have a strong feeling either way, but I
> think it would be clearer to use "bytes" instead of "pages" as argument.
> The reason is that with pages you are never sure of the actual
> granularity. Is it 4K? 16K? 64K? Especially considering that hypervisor
> pages can be of different size than guest pages. In theory you could
> have a situation where Xen uses 4K, Dom0 uses 16K and domU uses 64K, or
> any combination of the three. With bytes, at least you know the actual
> size.
> 
> If we use "bytes" as argument, then it also makes sense not to use the
> word "pages" in the hypercall name.
> 
> That said, any name would work and both bytes and pages would work, so
> I would leave it to the contributor who is doing the work to choose.

FAOD: There was no suggestion to use "pages" in the name; it was "paging"
which was suggested.

Jan