Re: [PATCH v3 0/7] iommu: Allow IOVA rcache range be configured

[Lu Baolu]

On 6/1/21 10:29 PM, John Garry wrote:
> For streaming DMA mappings involving an IOMMU and whose IOVA len regularly
> exceeds the IOVA rcache upper limit (meaning that they are not cached),
> performance can be reduced.
>
> This is much more pronounced from commit 4e89dce72521 ("iommu/iova: Retry
> from last rb tree node if iova search fails"), as discussed at [0].
>
> IOVAs which cannot be cached are highly involved in the IOVA ageing issue,
> as discussed at [1].
>
> This series allows the IOVA rcache range be configured, so that we may
> cache all IOVAs per domain, thus improving performance.
>
> A new IOMMU group sysfs file is added - max_opt_dma_size - which is used
> indirectly to configure the IOVA rcache range:
> /sys/kernel/iommu_groups/X/max_opt_dma_size
>
> This file is updated same as how the IOMMU group default domain type is
> updated, i.e. must unbind the only device in the group first.

Could you explain why it requires singleton group and driver unbinding
if the user only wants to increase the upper limit? I haven't dived into
the details yet, sorry if this is a silly question.

Best regards,
baolu