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

[Pranjal Shrivastava <praan@google.com>]

On Thu, Jul 02, 2026 at 04:38:55PM +0100, Kiryl Shutsemau wrote:
> 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.
>

Looks good. Thanks!

Praan