Re: [PATCH] iommu/io-pgtable-arm: Remove split on unmap behavior

[Will Deacon <will@kernel.org>]

On Fri, Nov 01, 2024 at 10:40:05AM -0300, Jason Gunthorpe wrote:
> On Fri, Oct 18, 2024 at 02:19:26PM -0300, Jason Gunthorpe wrote:
> > Of the page table implementations (AMD v1/2, VT-D SS, ARM32, DART)
> > arm_lpae is unique in how it handles partial unmap of large IOPTEs.
> > 
> > All other drivers will unmap the large IOPTE and return it's length.  For
> > example if a 2M IOPTE is present and the first 4K is requested to be
> > unmapped then unmap will remove the whole 2M and report 2M as the result.
> > 
> > arm_lpae instead replaces the IOPTE with a table of smaller IOPTEs, unmaps
> > the 4K and returns 4k. This is actually an illegal/non-hitless operation
> > on at least SMMUv3 because of the BBM level 0 rules.
> > 
> > Long ago VFIO could trigger a path like this, today I know of no user of
> > this functionality.
> > 
> > Given it doesn't work fully correctly on SMMUv3 and would create
> > portability problems if any user depends on it, remove the unique support
> > in arm_lpae and align with the expected iommu interface.
> > 
> > Outside the iommu users, this will potentially effect io_pgtable users of
> > ARM_32_LPAE_S1, ARM_32_LPAE_S2, ARM_64_LPAE_S1, ARM_64_LPAE_S2, and
> > ARM_MALI_LPAE formats.
> > 
> > Cc: Boris Brezillon <boris.brezillon@collabora.com>
> > Cc: Steven Price <steven.price@arm.com>
> > Cc: Liviu Dudau <liviu.dudau@arm.com>
> > Cc: dri-devel@lists.freedesktop.org
> > Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> > ---
> >  drivers/iommu/io-pgtable-arm.c | 72 +++-------------------------------
> >  1 file changed, 6 insertions(+), 66 deletions(-)
> 
> Updated commit message - Will let me know if you want me to resend
> with this, or any changes:

Thanks! Please send a new version with this text, the WARN we discussed
in the other part of the thread and also attacking the v7s code.

Cheers,

Will