Re: swiotlb/caamjr regression (Was: [GIT PULL] (swiotlb) stable/for-linus-5.12)

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, Jun 10, 2021 at 11:21 PM Christoph Hellwig <hch@lst.de> wrote:
>
> FYI, there has been a patch on the list that should have fixed this
> for about a month:
>
> https://lore.kernel.org/linux-iommu/20210510091816.GA2084@lst.de/T/#m0d0df6490350a08dcc24c9086c8edc165b402d6f

Honestly, that patch is all kinds of strange.

This expression:

    tlb_offset = (tlb_addr & (IO_TLB_SIZE - 1)) -
        swiotlb_align_offset(dev, orig_addr);

makes no sense to me. Maybe it happens to work, but I think it does so
by mistake rather than by design.

What my patch used was:

    unsigned long offset = (tlb_addr - mem->start) & (IO_TLB_SIZE - 1);

which actually pairs with - and makes sense with - the index calculation:

    int index = (tlb_addr - mem->start) >> IO_TLB_SHIFT;

so that offset truly is the offset within that index. Look at how that
'index' calculation calculates the high bits of the difference, and
the 'offset' calculation now literally is the low bits of the same
thing that got dropped on the floor by the 'index' calculation?

So those two calculations actually make sense. The
swiotlb_align_offset() one doesn't.

It's possible that that swiotlb_align_offset() function ends up giving
the right answer just almost by mistake (because of how tlb_addr and
orig_addr end up being related - the swiotlb_align_offset() expression
might just end up being the same thing - I didn't look deeper), but
even if the result is the same, it's not _sensible_ code,

It's also possible that the swiotlb_align_offset() function ends up
giving the right answer very much by design and because of how
orig_addr works - because maybe the remapping is doing odd things and
using that swiotlb_align_offset() function in ways that make the
*obvious* and natural offset calculation not actually work.

So it's at least in theory possible that my "natural offset"
calculation that matches how the index is calculated doesn't actually
work. But that means that the swiotlb remapping is doing some really
odd things, and then I think the patch would need a lot more
commentary on exactly what those very odd things are.

            Linus