Re: [PATCH v5] iommu/io-pgtable-arm: Add support for contiguous hint bit

[Robin Murphy]

On 09/10/2026 11:09 am, Vijayanand Jitta wrote:
> On 9/25/2026 4:23 AM, Jason Gunthorpe wrote:
> > On Thu, Sep 24, 2026 at 11:49:50AM -0700, Daniel Mentz wrote:
> > > On Wed, Sep 23, 2026 at 5:15 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > > > On Mon, Sep 21, 2026 at 04:44:07PM +0530, Vijayanand Jitta wrote:
> > > > > From: Prakash Gupta <prakash.gupta@xxxxxxxxxxxxxxxx>
> > > > >
> > > > > Add support for the contiguous hint (CONT) bit in ARM LPAE page tables.
> > > > > When a set of consecutive PTEs map a naturally aligned contiguous block of
> > > > > memory, set CONT on every descriptor in that group so the hardware can
> > > > > combine translations and improve TLB reach.
> > > > >
> > > > > Advertise the supported CONT group sizes in pgsize_bitmap. Callers select
> > > > > those sizes through the normal page-size selection path; io-pgtable-arm
> > > > > then installs the corresponding tagged descriptors directly. A partial
> > > > > unmap of a tagged CONT group is rejected before modifying any descriptor,
> > > > > so a rejected request cannot leave the group partly unmapped.
> > > > >
> > > > > The IO_PGTABLE_QUIRK_ARM_NO_CONT_HINT quirk allows SMMU drivers to disable
> > > > > CONT support for hardware with implementation-specific errata.
> > > >
> > > > smmuv3 has this errata, it must be disabled there too. I didn't notice
> > > > it in this patch?
> > >
> > > SMMU is an architecture specification. I am not aware of errors in
> > > this architecture specification that would preclude the usage of the
> > > contiguous bit. I understand that Arm MMU-700 has the following
> > > erratum
> > >
> > > 3777127 Under invalidation in TBU possible when using contiguous page
> > > table entries
> >
> > And a neoverse one too.
> >
> > > The recommended workaround is described as
> > >
> > > "Ensure that contiguous page tables are removed using a single range
> > > invalidation. Arm recommends using range invalidations to remove
> > > contiguous entries anyway for performance reasons."
> > >
> > > and I believe we are already doing this.
> >
> > No we aren't. Go read my fix on this:
> >
> > https://lore.kernel.org/linux-iommu/1-v7-e84261bbe7cd+2ea80b-smmu_tlbi_jgg@xxxxxxxxxx/
> >
> > I have another patch that fixes it for iommu domain mappings too.
> >
> > Jason
>
> Sure , will disable it for smmuv3 for now.

I still can't see how that would be necessary. Sure SVA has to cope with 
invalidating any old arbitarily-sized range that could have been a mix 
of pages, blocks, cont, whatever - that's fair enough. But for 
io-pgtable through the IOMMU API, a partial unmap of anyhthing which 
could have been mapped as a cont range would already be invalid and 
should fail. Thus for any unmap which could validly include any cont 
ranges, iommu_pgsize() would have already picked a granularity that is 
some multiple of the largest cont range size being unmapped, so even if 
the total gathered size exceeds a single command, we still wouldn't 
split it _within_ any single one of those ranges, only at a boundary 
between two unrelated ones.

Thanks,
Robin.