Re: [PATCH v7 2/2] iommu/arm-smmu-v3: Default queue depths to one page in a kdump kernel

[Breno Leitao]

On Fri, Sep 25, 2026 at 03:15:30PM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
> 
> All three queues are sized from the maxima the hardware advertises in IDR1
> and allocated at probe, up to 4 MB each on a 4K-page kernel. The capture
> kernel already disables two of them: arm_smmu_device_reset() drops
> CR0_EVTQEN and CR0_PRIQEN. It still allocates both at full size.
> 
> A kdump capture kernel runs from a small crashkernel reservation, and every
> SMMUv3 instance pays that cost again, up to 12 MB apiece. It goes to queues
> that either serve the handful of devices used to save the dump or are
> switched off outright, and it is memory the dump itself needs.
> 
> Size all three queues at one page worth of entries when is_kdump_kernel().
> The queues carry commands and fault records rather than DMA data, 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 few
> devices that save the dump. The cmdq_max_n_shift parameter therefore does
> not apply in a capture kernel.
> 
> Suggested-by: Kyle McMartin <jkkm@xxxxxxxx>
> Assisted-by: LLM
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Tested-by: Yuanhe Shu <xiangzao@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>

Reviewed-by: Breno Leitao <leitao@xxxxxxxxxx>