Re: [PATCH v4] iommu: Optimise PCI SAC address trick

[Robin Murphy <robin.murphy@arm.com>]

On 23/05/2023 5:06 pm, Joerg Roedel wrote:
> On Fri, Apr 14, 2023 at 06:45:57PM +0100, Robin Murphy wrote:
>> Sounds good - I'm considerably more confident in this approach, but although
>> it should not be able to break any scenario which wasn't already broken, it
>> could potentially still make such a breakage more noticeable. Thus in all
>> honesty I'd feel happiest giving it a full cycle of -next coverage as well.
> 
> I had some second thoughts on this, wouldn't it be better to change the
> allocator to allocate from lowest addresses first? Then we can just
> remove the SAC trick and rely on dma-masks only.
> 
> Thoughts?

Yes, in the long term I definitely would like to have a more flexible 
allocator - this is more of a stop-gap measure that's an easy win with 
what we have now.

Top-down allocation is nice in that it makes for easily recognisable DMA 
addresses, and does do a great job of flushing out bugs, but having the 
option of bottom-up allocation would definitely be useful in various 
cases - realistically it's pretty much a prerequisite for converting 
arch/arm to use iommu-dma. However, given all the other scalability 
issues that keep coming to light, I think that's going to be the tipping 
point for ripping up the existing code and giving it a major overhaul, 
which I would love to be able to get stuck in to, but don't have the 
capacity for at the moment.

Thanks,
Robin.