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

[Joerg Roedel]

Hi Robin,

On Thu, Apr 13, 2023 at 02:40:25PM +0100, Robin Murphy wrote:
> Per the reasoning in commit 4bf7fda4dce2 ("iommu/dma: Add config for
> PCI SAC address trick") and its subsequent revert, this mechanism no
> longer serves its original purpose, but now only works around broken
> hardware/drivers in a way that is unfortunately too impactful to remove.
> 
> This does not, however, prevent us from solving the performance impact
> which that workaround has on large-scale systems that don't need it.
> Once the 32-bit IOVA space fills up and a workload starts allocating and
> freeing on both sides of the boundary, the opportunistic SAC allocation
> can then end up spending significant time hunting down scattered
> fragments of free 32-bit space, or just reestablishing max32_alloc_size.
> This can easily be exacerbated by a change in allocation pattern, such
> as by changing the network MTU, which can increase pressure on the
> 32-bit space by leaving a large quantity of cached IOVAs which are now
> the wrong size to be recycled, but also won't be freed since the
> non-opportunistic allocations can still be satisfied from the whole
> 64-bit space without triggering the reclaim path.
> 
> However, in the context of a workaround where smaller DMA addresses
> aren't simply a preference but a necessity, if we get to that point at
> all then in fact it's already the endgame. The nature of the allocator
> is currently such that the first IOVA we give to a device after the
> 32-bit space runs out will be the highest possible address for that
> device, ever. If that works, then great, we know we can optimise for
> speed by always allocating from the full range. And if it doesn't, then
> the worst has already happened and any brokenness is now showing, so
> there's little point in continuing to try to hide it.
> 
> To that end, implement a flag to refine the SAC business into a
> per-device policy that can automatically get itself out of the way if
> and when it stops being useful.

Thanks for working on this, I think this is good to go. But given the
issues we had with last attempt I'd like to have this in linux-next for
a few weeks before sending it upstream. Therefore I will defer this
patch and merge it early in the next cycle.

Regards,

	Joerg