Re: [PATCH v4 6/6] xen/ppc: mm-radix: Allocate all paging structures at runtime

[Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>]

Hi Jan,

I'm revisiting this series and wanted to follow up on your comments.

On 4/24/24 9:46 AM, Jan Beulich wrote:
> On 12.04.2024 05:55, Shawn Anastasio wrote:
>> +/*
>> + * A thin wrapper for alloc_boot_pages that keeps track of the maximum and
>> + * minimum mfns that have been allocated. This information is used by
>> + * setup_initial_mapping to include the allocated pages in the initial
>> + * page mapping.
> 
> Plus everything in between. Together with the further comment below I'm
> afraid I still think that the constraints (and the justification for them
> being tolerable at least for the time being) aren't made sufficiently
> clear (perhaps in another code comment next to the respective two
> variables' definitions).
> 

Sure, I'll elaborate a bit further in a new comment above those two
variables.

>> +    /*
>> +     * Identity map all pages we've allocated for paging structures. This 
>> act
>> +     * itself will allocate more pages, so continue until we've mapped from
>> +     * `max_alloc_mfn` down to `min_alloc_mfn`. This assumes that the heap 
>> grows
>> +     * downwards, which matches the behavior of alloc_boot_pages.
> 
> ... as long as you only ever allocate individual pages (which looks
> to be the case for such incremental allocations, but that imo doesn't
> go without saying).
> 

I'm not sure I'm following you. `initial_page_alloc` as well as its
underlying `alloc_boot_pages` can only ever allocate individual pages
(or contiguous ranges of pages). If the assumption about growing
downwards is not met, the code catches it and panics.

> Jan

Thanks,
Shawn