Re: [PATCH v2 00/11] iommu/arm-smmu-v3: Add PRI support

[harsha.v@oss.qualcomm.com]

Hi Nicolin Chen,

On 5/28/2026 1:29 PM, Nicolin Chen wrote:
> The SMMUv3 driver doesn't handle events on the PRI queue or respond to IOPF
> faults. This series adds the missing pieces, using the IOPF infrastructure,
> to convert PRI page requests into iopf_faults and issue CMDQ_OP_PRI_RESP.
> 
> The iopf_queue_flush_dev() contract requires the driver to first drain the
> hardware PRI queue and synchronize using a threaded IRQ handler before the
> IOPF software flush. This drove the additional commits compared to v1:
>   - arm_smmu_drain_queue_for_iopf() drains the hardware queue to the PROD
>     snapshot
>   - arm_smmu_attach_release() moves the teardown outside the global lock
>   - synchronize_irq() closes the gap before the final flush
> 
> This is on Github:
> https://github.com/nicolinc/iommufd/commits/smmuv3_pri-v2
> 
> FWIW, engineers on the NVIDIA side have managed to verify the PRI feature.
> 
> Changelog
> v2:
>   * Allocate evtq.iopf for ARM_SMMU_FEAT_PRI
>   * Pick up Jean's PRI stubs and PRI export patches
>   * Enable PRI for PCI devices in arm_smmu_probe_device()
>   * Add arm_smmu_drain_queue_for_iopf() for EVTQ and PRIQ
>   * Add arm_smmu_attach_release() to rework the IOPF drain
>   * Add IOMMU_FAULT_PAGE_REQUEST_STALLS_TRANS for STALL mode
>   * Gate pci_enable_pri() on FEAT_PRI plus a non-NULL evtq.iopf
>   * Deny unrecognised-StreamID PRG_LAST in arm_smmu_handle_ppr()
>   * Disable PRI when no IRQ handler is registered (unique or combined IRQ)
> v1:
>   https://lore.kernel.org/all/cover.1772568590.git.nicolinc@nvidia.com/
> 
> Jean-Philippe Brucker (2):
>    PCI/ATS: Add PRI stubs
>    PCI/ATS: Export pci_enable_pri() and pci_reset_pri()
> 
> Malak Marrid (1):
>    iommu/arm-smmu-v3: Submit CMDQ_OP_PRI_RESP for IOPF event
> 
> Nicolin Chen (8):
>    iommu/arm-smmu-v3: Add arm_smmu_attach_release()
>    iommu/arm-smmu-v3: Factor out __queue_empty() and __queue_consumed()
>    iommu/arm-smmu-v3: Add arm_smmu_drain_queue_for_iopf() helper
>    iommu/arm-smmu-v3: Drain in-flight fault handlers
>    iommu/arm-smmu-v3: Support PRI Page Request in arm_smmu_handle_ppr()
>    iommu/arm-smmu-v3: Disable PRI when no IRQ handler is registered
>    iommu/arm-smmu-v3: Allocate IOPF queue for ARM_SMMU_FEAT_PRI
>    iommu/arm-smmu-v3: Enable PRI for PCI device in
>      arm_smmu_probe_device()
> 
>   drivers/iommu/arm/Kconfig                     |   1 +
>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h   |   3 +
>   include/linux/iommu.h                         |   1 +
>   include/linux/pci-ats.h                       |   5 +
>   .../arm/arm-smmu-v3/arm-smmu-v3-iommufd.c     |   1 +
>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c   | 244 +++++++++++++++---
>   drivers/pci/ats.c                             |   2 +
>   7 files changed, 221 insertions(+), 36 deletions(-)
> 
> 
> base-commit: 74fa4c177ad09800b007cba043370c887bb1b4e3

One thing I noticed while reviewing: when arm_smmu_priq_thread() detects
PRIQ overflow, partial faults (non-LAST pages stored via
report_partial_fault()) whose LAST page was lost in the overflow remain
permanently in iopf_param->partial. This is a monotonic memory leak —
it grows with each overflow event.

Intel VT-d handles this in prq_event_thread() (drivers/iommu/intel/prq.c):
     if (head == tail) {
         iopf_queue_discard_partial(iommu->iopf_queue);
         writel(DMA_PRS_PRO, iommu->reg + DMAR_PRS_REG);
     }

iopf_queue_discard_partial() was written for exactly this scenario.
Could we add the same here arm_smmu_priq_thread()
(drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c) ?

     if (queue_sync_prod_in(q) == -EOVERFLOW) {
         dev_err(smmu->dev, "PRIQ overflow detected -- requests lost\n");
+       iopf_queue_discard_partial(smmu->evtq.iopf);
     }

At this point all surviving entries have already been consumed by the
loop above, so discarding unconditionally is safe — implicitly matching
Intel's "head == tail" guard.

Best Regards,
Harsha Vardhan V