Re: [PATCH 20/24] irqchip/gic-v5: Add GICv5 LPI/IPI support

[Thomas Gleixner <tglx@linutronix.de>]

On Fri, Apr 11 2025 at 11:26, Lorenzo Pieralisi wrote:
> On Tue, Apr 08, 2025 at 12:50:19PM +0200, Lorenzo Pieralisi wrote:
>> Maple tree entries are not used by the driver, only the range tracking
>> is required - therefore the driver first finds an empty area large
>> enough to contain the required number of LPIs then checks the
>> adjacent (and possibly occupied) LPI ranges and try to merge them
>> together, reducing maple tree slots usage.
>
> The maple tree usage for this purpose is an RFC at this stage.
>
> Added Alexei because I know BPF arena used the maple tree in
> a similar way in the past and moved to a range tree because
> the BPF arena requires a special purpose mem allocator.
>
> As Thomas already pointed out a plain bitmap could do even though
> it requires preallocating memory up to 2MB (or we can grow it
> dynamically).
>
> We could allocate IDs using an IDA as well, though that's 1 by 1,
> we allocate LPI INTIDs 1 by 1 - mostly, upon MSI allocation, so
> using an IDA could do (AFAIU it works for 0..INT_MAX we need
> 0..2^24 worst case).

The point is that you really only need a 1-bit storage per entry,
i.e. used/unused. You won't use any of the storage functions of maple
tree, idr or whatever.

So the obvious choice is a bitmap and as you said, it's trivial to start
with a reasonably sized one and reallocate during runtime if the need
arises.

The reallocation happens in domain::ops::alloc() which is fully
preemptible context, i.e. no restrictions vs. allocations.

For the top-most domain, the callers hold domain::mutex, which excludes
concurrency vs. ops::alloc/free(). If the bitmap is in a domain further
down the hierarchy then you need your own mutex there.

Thanks,

       tglx