Re: properly communicate queue limits to the DMA layer

From: Jens Axboe
Date: Wed Jun 05 2019 - 15:21:24 EST

On 6/5/19 1:08 PM, Christoph Hellwig wrote:
> Hi Jens,
> we've always had a bit of a problem communicating the block layer
> queue limits to the DMA layer, which needs to respect them when
> an IOMMU that could merge segments is used. Unfortunately most
> drivers don't get this right. Oddly enough we've been mostly
> getting away with it, although lately dma-debug has been catching
> a few of those issues.
> The segment merging fix for devices with PRP-like structures seems
> to have escalated this a bit. The first patch fixes the actual
> report from Sebastian, while the rest fix every drivers that appears
> to have the problem based on a code audit looking for drivers using
> blk_queue_max_segment_size, blk_queue_segment_boundary or
> blk_queue_virt_boundary and calling dma_map_sg eventually. Note
> that for SCSI drivers I've taken the blk_queue_virt_boundary setting
> to the SCSI core, similar to how we did it for the other two settings
> a while ago. This also deals with the fact that the DMA layer
> settings are on a per-device granularity, so the per-device settings
> in a few SCSI drivers can't actually work in an IOMMU environment.
> It would be nice to eventually pass these limits as arguments to
> dma_map_sg, but that is a far too big series for the 5.2 merge
> window.

Since I'm heading out shortly and since I think this should make
the next -rc, I'll tentatively queue this up.

Jens Axboe