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

[Will Deacon]

On Mon, Jul 06, 2026 at 09:47:08AM +0100, Kiryl Shutsemau (Meta) wrote:
> All SMMU queues are sized from the maxima the hardware advertises in IDR1,
> which can be several megabytes each, and are allocated at probe. The kdump
> kernel already disables the event and PRI queues (arm_smmu_device_reset()
> drops CR0_EVTQEN/CR0_PRIQEN) but still allocates them at full size. 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 are not on the DMA data path, so dump throughput is
> unaffected; a shallower command 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.

Wouldn't we be better of not allocating unused queues in the first place?
That's what this patch does:

https://lore.kernel.org/r/0b035c53cb401acde8244b805d4b6a0312b83708.1782799827.git.nicolinc@xxxxxxxxxx

so help reviewing that series would be much appreciated!

If you want to reduce the cmdq size, I'd prefer a cmdline option rather
than special-casing kdump (as I've had other folks ask about configuring
the cmdq size for other reasons).

Will