linux-kernel - Re: properly communicate queue limits to the DMA layer

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Date:   Wed, 5 Jun 2019 13:17:15 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Christoph Hellwig <hch@....de>
Cc:     Sebastian Ott <sebott@...ux.ibm.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Max Gurtovoy <maxg@...lanox.com>,
        Bart Van Assche <bvanassche@....org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Oliver Neukum <oneukum@...e.com>, linux-block@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-mmc@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-scsi@...r.kernel.org,
        megaraidlinux.pdl@...adcom.com, MPT-FusionLinux.pdl@...adcom.com,
        linux-hyperv@...r.kernel.org, linux-usb@...r.kernel.org,
        usb-storage@...ts.one-eyed-alien.net, linux-kernel@...r.kernel.org
Subject: Re: properly communicate queue limits to the DMA layer

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