linux-kernel - Re: [PATCH 1/3] swiotlb: Export maximum allocation size

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Message-ID: <20190115130536.GA28364@lst.de>
Date:   Tue, 15 Jan 2019 14:05:36 +0100
From:   Christoph Hellwig <hch@....de>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Joerg Roedel <joro@...tes.org>,
        Jason Wang <jasowang@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        virtualization@...ts.linux-foundation.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, jfehlig@...e.com,
        jon.grimm@....com, brijesh.singh@....com, hch@....de,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 1/3] swiotlb: Export maximum allocation size

On Mon, Jan 14, 2019 at 04:59:27PM -0500, Michael S. Tsirkin wrote:
> On Mon, Jan 14, 2019 at 03:49:07PM -0500, Konrad Rzeszutek Wilk wrote:
> > On Fri, Jan 11, 2019 at 10:12:31AM +0100, Joerg Roedel wrote:
> > > On Thu, Jan 10, 2019 at 12:02:05PM -0500, Konrad Rzeszutek Wilk wrote:
> > > > Why not use swiotlb_nr_tbl ? That is how drivers/gpu/drm use to figure if they
> > > > need to limit the size of pages.
> > > 
> > > That function just exports the overall size of the swiotlb aperture, no?
> > > What I need here is the maximum size for a single mapping.
> > 
> > Yes. The other drivers just assumed that if there is SWIOTLB they would use
> > the smaller size by default (as in they knew the limitation).
> > 
> > But I agree it would be better to have something smarter - and also convert the
> > DRM drivers to piggy back on this.
> > 
> > Or alternatively we could make SWIOTLB handle bigger sizes..
> 
> 
> Just a thought: is it a good idea to teach blk_queue_max_segment_size
> to get the dma size? This will help us find other devices
> possibly missing this check.

Yes, we should.  Both the existing DMA size communicated through dma_params
which is set by the driver, and this new DMA-ops exposed one which needs
to be added.  I'm working on some preliminary patches for the first part,
as I think I introduced a bug related to that in the SCSI layer in 5.0..