Re: [RFC PATCH 1/2] swiotlb: Split up single swiotlb lock

[Robin Murphy]

On 2022-04-28 15:55, Andi Kleen wrote:
> On 4/28/2022 7:45 AM, Christoph Hellwig wrote:
> > On Thu, Apr 28, 2022 at 03:44:36PM +0100, Robin Murphy wrote:
> > > Rather than introduce this extra level of allocator complexity, how 
> > > about
> > > just dividing up the initial SWIOTLB allocation into multiple io_tlb_mem
> > > instances?
> > Yeah.  We're almost done removing all knowledge of swiotlb from drivers,
> > so the very last thing I want is an interface that allows a driver to
> > allocate a per-device buffer.
>
> At least for TDX need parallelism with a single device for performance.
>
> So if you split up the io tlb mems for a device then you would need a 
> new mechanism to load balance the requests for single device over those. 
> I doubt it would be any simpler.

Eh, I think it would be, since the round-robin retry loop can then just 
sit around the existing io_tlb_mem-based allocator, vs. the churn of 
inserting it in the middle, plus it's then really easy to statically 
distribute different starting points across different devices via 
dev->dma_io_tlb_mem if we wanted to.

Admittedly the overall patch probably ends up about the same size, since 
it likely pushes a bit more complexity into swiotlb_init to compensate, 
but that's still a trade-off I like.

Thanks,
Robin.