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

[Jason Gunthorpe <jgg@ziepe.ca>]

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 <jkkm@meta.com>
> 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