Re: [PATCH v3] iommu: Optimise PCI SAC address trick

[Robin Murphy]

On 2023-04-13 11:16, John Garry wrote:
> On 20/01/2023 11:33, John Garry wrote:
> > On 18/01/2023 17:26, 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 the workaround imposes on large-scale systems that don't need it.
> > > That is felt once the 32-bit IOVA space fills up and we keep
> > > unsuccessfully trying to allocate from it. However, if we get to that
> > > point then in fact it's already the endgame. The nature of the allocator
> > > is 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 can be pretty sure it's safe to 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 no point continuing to try to hide it.
> > >
> > > To that end, implement a flag to refine this into a per-device policy
> > > that can automatically get itself out of the way if and when it stops
> > > being useful.
> > >
> > > CC: John Garry <john.garry@xxxxxxxxxx>
> > > CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> > > ---
> > >
> > > v3: Expand the flag name, add a print with inline commentary for good
> > >      measure, and refactor the code flow even more (too many ifs and
> > >      indents...) such that I didn't presume to carry forward John's R-b.
> >
> > I like the new changes, so feel free to add:
> >
> > Reviewed-by: John Garry <john.g.garry@xxxxxxxxxx>
>
> Is there any chance that this can be picked up?
>
> I also saw that it fixed an issue for Jakub (cc'ed) recently.

Oh, thanks for the reminder - IIRC this wants a minor rebase now, let me 
double-check and send a v4 that applies cleanly...

Cheers,
Robin.