Re: [PATCH v4] io: add io_pgtable abstraction

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Dec 19, 2025 at 10:05:57AM -0400, Jason Gunthorpe wrote:
> On Fri, Dec 19, 2025 at 10:50:52AM +0000, Alice Ryhl wrote:
> > +// For now, we do not provide the ability to flush the TLB via the built-in callback mechanism.
> > +// Instead, the `map_pages` function requires the caller to explicitly flush the TLB before the
> > +// pgtable is used to access the newly created range.
> > +//
> > +// This is done because the initial user of this abstraction may perform many calls to `map_pages`
> > +// in a single batched operation, and wishes to only flush the TLB once after performing the entire
> > +// batch of mappings. These callbacks would flush too often for that use-case.
> > +//
> > +// Support for flushing the TLB in these callbacks may be added in the future.
> > +static NOOP_FLUSH_OPS: bindings::iommu_flush_ops = bindings::iommu_flush_ops {
> > +    tlb_flush_all: Some(rust_tlb_flush_all_noop),
> > +    tlb_flush_walk: Some(rust_tlb_flush_walk_noop),
> > +    tlb_add_page: None,
> > +};
> 
> This comment seems quite off..
> 
> Usually you don't flush on map, you flush on unmap. The TLB should be
> empty upon mapping and not need flushing - except for the rarer
> special cases of clearing the walk cache which cannot be detected any
> other way than using these callbacks. Doing a big flush on map to deal
> with the walk cache would be worse than implementing these callbacks.
> 
> The flush on unmap, at least for ARM style invalidations, needs these
> callbacks because they provide required information. If the actual HW
> does not use an ARM style invalidation system then this page table
> code is not optimal for it.

You should not assume that the way I worded something implies that the
GPU hardware does something weird. It's more likely that I just got
something wrong.

It looks like panthor / tyr flush the range that was modified after both
map and unmap operations.

Alice