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

[Nicolin Chen]

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 at intel.com>
Suggested-by: Jason Gunthorpe <jgg at nvidia.com>
Suggested-by: Kiryl Shutsemau (Meta) <kas at kernel.org>
Suggested-by: Will Deacon <will at kernel.org>
Link: https://lore.kernel.org/all/amiBagGKn-Aym1DK@willie-the-truck/
Assisted-by: Claude:claude-opus-5
Signed-off-by: Nicolin Chen <nicolinc at nvidia.com>
---
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
index c776fdd43325d..7b9ef1ad1f688 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -4981,9 +4981,6 @@ static int arm_smmu_device_reset(struct arm_smmu_device *smmu)
 		return ret;
 	}
 
-	if (is_kdump_kernel())
-		enables &= ~(CR0_EVTQEN | CR0_PRIQEN);
-
 	/* Enable the SMMU interface */
 	enables |= CR0_SMMUEN;
 	ret = arm_smmu_write_reg_sync(smmu, enables, ARM_SMMU_CR0,
@@ -5296,6 +5293,15 @@ static int arm_smmu_device_hw_probe(struct arm_smmu_device *smmu)
 	if (arm_smmu_sva_supported(smmu))
 		smmu->features |= ARM_SMMU_FEAT_SVA;
 
+	/*
+	 * A kdump kernel wants neither queue: the crashed kernel's CDs and page
+	 * tables might be corrupted, spamming events, and page requests cannot
+	 * be served. A disabled queue discards new records without raising any
+	 * global error.
+	 */
+	if (is_kdump_kernel())
+		smmu->features &= ~(ARM_SMMU_FEAT_EVTQ | ARM_SMMU_FEAT_PRI);
+
 	dev_info(smmu->dev, "oas %lu-bit (features 0x%08x)\n",
 		 smmu->oas, smmu->features);
 	return 0;
-- 
2.43.0