Re: [PATCH v3] mm/pdx: Add comments throughout the codebase for pdx

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.07.2023 12:27, Alejandro Vallejo wrote:
> On Thu, Jul 13, 2023 at 05:12:09PM +0200, Jan Beulich wrote:
>> On 07.07.2023 18:07, Alejandro Vallejo wrote:
>>> --- a/xen/include/xen/mm.h
>>> +++ b/xen/include/xen/mm.h
>>> @@ -31,6 +31,17 @@
>>>   *   (i.e. all devices assigned to) a guest share a single DMA address 
>>> space
>>>   *   and, by default, Xen will ensure dfn == pfn.
>>>   *
>>> + * pdx: Page InDeX
>>> + *   Indices into the frame table holding the per-page's book-keeping
>>> + *   metadata. A compression scheme may be used, so there's a possibly non
>>> + *   identity mapping between valid(mfn) <-> valid(pdx). See the comments
>>> + *   in pdx.c for an in-depth explanation of that mapping. This also has a
>>> + *   knock-on effect on the directmap, as "compressed" pfns have no
>>> + *   corresponding mapped frames.
>>
>> Didn't you mean to keep the directmap part optional,
> I did.
> 
>> which would call for saying "may" here (twice)?
> That paragraph as-is doesn't really mandate a directmap. It merely state
> that there are knock-on effects on directmap indexing, not that there's
> always a directmap to index.
> 
>> Yet then ...
>>
>>
>>> --- a/xen/include/xen/pdx.h
>>> +++ b/xen/include/xen/pdx.h
>>> @@ -1,6 +1,73 @@
>>>  #ifndef __XEN_PDX_H__
>>>  #define __XEN_PDX_H__
>>>  
>>> +/*
>>> + * PDX (Page inDeX)
>>> + *
>>> + * This file deals with optimisations pertaining to frame table and
>>> + * directmap indexing, A pdx is an index into the frame table, which
>>> + * typically also means an index into the directmap[1]. However, having an
>>> + * identity relationship between mfn and pdx could waste copious amounts of
>>> + * memory in empty frame table entries and page tables. There are some
>>> + * techniques to bring memory wastage down.
>>> + *
>>> + * [1] Some ports apply further modifications to a pdx before indexing the
>>> + *     directmap. This doesn't change the fact that the same compression
>>> + *     present in the frame table is also present in the directmap
>>> + *     whenever said map is present.
>>
>> .. you mention it here as non-optional as well. Didn't you tell me that
>> Arm doesn't use compressed indexes into the directmap?
>>
>> Jan
> 
> The [1] note states "whenever said map is present", meaning that it may not
> be present. Saying it's optional is a stretch though. It's not like we can
> choose right now.
> 
>> Didn't you tell me that Arm doesn't use compressed indexes into the 
>> directmap?
> arm32 doesn't have a directmap at all. arm64 uses biased pdx as indices
> (they are offset by a constant), so they are still subject to compression.

Hmm, then our understanding of "optional" was differing: I understood
"use of compressed indexes is optional", when you apparently meant
"the use of a directmap is optional". If this is the case, then I
agree with the chosen wording. (Nevertheless using the suggested "may"
wouldn't yield and incorrect outcome.)

Jan