Re: [PATCH 1/8] iommu: Lift and generalize the STE/CD update code from SMMUv3

[Will Deacon <will@kernel.org>]

On Sat, Mar 14, 2026 at 04:13:27PM +0800, Baolu Lu wrote:
> On 3/10/26 08:06, Samiullah Khawaja wrote:
> > On Mon, Mar 09, 2026 at 11:33:23PM +0000, Samiullah Khawaja wrote:
> > > On Mon, Mar 09, 2026 at 02:06:41PM +0800, Lu Baolu wrote:
> > > > From: Jason Gunthorpe <jgg@nvidia.com>
> > > > 
> > > > Many IOMMU implementations store data structures in host memory that can
> > > > be quite big. The iommu is able to DMA read the host memory using an
> > > > atomic quanta, usually 64 or 128 bits, and will read an entry using
> > > > multiple quanta reads.
> > > > 
> > > > Updating the host memory datastructure entry while the HW is
> > > > concurrently
> > > > DMA'ing it is a little bit involved, but if you want to do this
> > > > hitlessly,
> > > > while never making the entry non-valid, then it becomes quite
> > > > complicated.
> > > > 
> > > > entry_sync is a library to handle this task. It works on the notion of
> > > > "used bits" which reflect which bits the HW is actually sensitive to and
> > > > which bits are ignored by hardware. Many hardware specifications say
> > > > things like 'if mode is X then bits ABC are ignored'.
> > > > 
> > > > Using the ignored bits entry_sync can often compute a series of ordered
> > > > writes and flushes that will allow the entry to be updated while keeping
> > > > it valid. If such an update is not possible then entry will be made
> > > > temporarily non-valid.
> > > > 
> > > > A 64 and 128 bit quanta version is provided to support existing iommus.
> > > > 
> > > > Co-developed-by: Lu Baolu <baolu.lu@linux.intel.com>
> > > > Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> > > > Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> > > > ---
> > > > drivers/iommu/Kconfig               |  14 +++
> > > > drivers/iommu/Makefile              |   1 +
> > > > drivers/iommu/entry_sync.h          |  66 +++++++++++++
> > > > drivers/iommu/entry_sync_template.h | 143 ++++++++++++++++++++++++++++
> > > > drivers/iommu/entry_sync.c          |  68 +++++++++++++
> > > > 5 files changed, 292 insertions(+)
> > > > create mode 100644 drivers/iommu/entry_sync.h
> > > > create mode 100644 drivers/iommu/entry_sync_template.h
> > > > create mode 100644 drivers/iommu/entry_sync.c

Shouldn't we move the SMMU driver over to this, rather than copy-pasting
everything? If not, then why is it in generic IOMMU code?

Will