Re: [PATCH] iommu/arm-smmu-v3: Shrink command/event/PRI queues in kdump kernel

On Wed, Jul 01, 2026 at 04:45:28PM +0100, Kiryl Shutsemau (Meta) wrote:

The command, event and PRI queues are sized from the maxima the hardware
advertises in IDR1, which can be several megabytes each. On systems with
many SMMUv3 instances that cost is paid per instance and adds up to tens
of megabytes of coherent DMA in the capture kernel.

A kdump capture kernel runs from a small crashkernel reservation and only
has to drive the few devices used to save the dump, so deep queues serve
no purpose. The queues carry invalidation commands and fault records, not
DMA data, so dump throughput is unaffected; a shallower queue only bounds
how many commands may be in flight before a sync, which does not matter for
the capture kernel's small device count and modest I/O.

Clamp every queue to a single page when is_kdump_kernel() is true. Doing
it in arm_smmu_init_one_queue() covers the command, event and PRI queues
in one place. The command queue still holds at least one batch plus a sync
(256 entries on a 4K-page kernel, well above CMDQ_BATCH_ENTRIES), so
command batching keeps working.

Suggested-by: Kyle McMartin <redacted>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

Make sense to me

+	if (is_kdump_kernel()) {
+		u32 ent_sz_shift = ilog2(dwords) + 3;
+
+		q->llq.max_n_shift = min_t(u32, q->llq.max_n_shift,
+					   PAGE_SHIFT - ent_sz_shift);

I saw lately many people saying you should not use min_t, why is it
needed here?

Jason