Re: [PATCH v10 04/13] iommu/arm-smmu-v3: Disable the EVTQ and the PRIQ in a kdump kernel

On Sun, Aug 30, 2026 at 04:18:05PM -0700, Nicolin Chen wrote:

A kdump kernel cannot use either queue. The crashed kernel's CDs and page
tables might be corrupted, so events would spam the EVTQ, and there is no
way to serve the page requests that would arrive at the PRIQ.

The reset routine still enables both of the queues and then masks the two
enable bits back out, having already programmed the queue bases and taken
the interrupts of both.

Clear ARM_SMMU_FEAT_EVTQ and ARM_SMMU_FEAT_PRI in the probe instead, so all
of the queue handling, i.e. the interrupts, the IRQ_CTRL bits and the CR0
enables, keeps away from a kdump kernel via the feature tests.

Both queues also go unallocated now, as the two allocations test the same
features. Each of them is sized from the maxima that IDR1 advertises, so up
to 4MB apiece on a 4K-page kernel, which a kdump kernel would otherwise pay
for every SMMU instance out of a small crashkernel reservation.

The IOPF workqueue of the event queue stays allocated here, as untangling
it from the SVA and the stall features takes a pair of changes carried by
the PRI series. It costs a struct and a workqueue rather than the megabytes
that the two queues take, so leave it to that series.

Suggested-by: Kevin Tian <kevin.tian@intel.com>
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Suggested-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Suggested-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/all/amiBagGKn-Aym1DK@willie-the-truck/ (local)
Assisted-by: Claude:claude-opus-5
Signed-off-by: Nicolin Chen <redacted>

Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov