Re: [PATCH v5 0/5] DMA mapping changes for SCSI core

[John Garry]

On 11/07/2022 00:08, Damien Le Moal wrote:
> > Ah, I think that I misunderstood Damien's question. I thought he was
> > asking why not keep shost max_sectors at dma_max_mapping_size() and then
> > init each sdev request queue max hw sectors at dma_opt_mapping_size().
> I was suggesting the reverse:)  Keep the device hard limit
> (max_hw_sectors) to the max dma mapping and set the soft limit
> (max_sectors) to the optimal dma mapping size.

Sure, but as I mentioned below, I only see a small % of requests whose 
mapping size exceeds max_sectors but that still causes a big performance 
hit. So that is why I want to set the hard limit as the optimal dma 
mapping size.

Indeed, the IOMMU IOVA caching limit is already the same as default 
max_sectors for the disks in my system - 128Kb for 4k page size.

> > But he seems that you want to know why not have the request queue max
> > sectors at dma_opt_mapping_size(). The answer is related to meaning of
> > dma_opt_mapping_size(). If we get any mappings which exceed this size
> > then it can have a big dma mapping performance hit. So I set max hw
> > sectors at this ‘opt’ mapping size to ensure that we get no mappings
> > which exceed this size. Indeed, I think max sectors is 128Kb today for
> > my host, which would be same as dma_opt_mapping_size() value with an
> > IOMMU enabled. And I find that only a small % of request size may exceed
> > this 128kb size, but it still has a big performance impact.

Thanks,
John