Re: [PATCH v2] iommu/arm-smmu-v3: Shrink command/event/PRI queues in kdump kernel

[Kiryl Shutsemau <kas@kernel.org>]

On Thu, Jul 02, 2026 at 03:05:12PM +0000, Pranjal Shrivastava wrote:
> On Thu, Jul 02, 2026 at 12:28:25PM +0100, Kiryl Shutsemau (Meta) wrote:
> > The command, event and PRI queues are sized from the maxima the hardware
> 
> A minor note here is PRI & EVT queues are disabled for the kdump kernel
> (see arm_smmu_device_reset). We could just mention all SMMU queues are
> sized [...] in the commit message. 

Fair enough.

Here's updated commit message (I will send v3 in few days, if no new
feedback):

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

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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov