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

[Breno Leitao <leitao@debian.org>]

On Wed, Jul 01, 2026 at 09:16:03PM -0300, Jason Gunthorpe wrote:
> 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?

Good point, it seems that both of them are u32

    - q->llq.max_n_shift is u32 (struct arm_smmu_ll_queue)
    - ent_sz_shift is u32, and PAGE_SHIFT is a small int constant, so
      PAGE_SHIFT - ent_sz_shift promotes to u32 too.

min() should be enough, I would say. With that, please feel free to add:

Reviewed-by: Breno Leitao <leitao@debian.org>