linux-kernel - Re: [PATCH 0/3] Fix virtio-blk issue with SWIOTLB

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Message-ID: <20190110142640.GA5825@8bytes.org>
Date:   Thu, 10 Jan 2019 15:26:40 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Christoph Hellwig <hch@....de>
Cc:     "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.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
Subject: Re: [PATCH 0/3] Fix virtio-blk issue with SWIOTLB

Hi Christoph,

On Thu, Jan 10, 2019 at 02:59:52PM +0100, Christoph Hellwig wrote:
> On Thu, Jan 10, 2019 at 02:44:30PM +0100, Joerg Roedel wrote:

> > The problem is a limitation of the SWIOTLB implementation,
> > which does not support allocations larger than 256kb. When
> > the virtio-blk driver tries to read/write a block larger
> > than that, the allocation of the dma-handle fails and an IO
> > error is reported.
> 
> s/allocations/mappings/, right?  We don't use the swiotlb
> buffer for the coherent allocations anymore, and I don't think
> virtio-blk uses them anyway.

'Allocation' in the sense that there are address ranges allocated, not
memory, but mappings would also be right.

> I really don't like the fact that we hardcode swiotlb specific.
> This needs to be indirect through the dma ops or struct device,
> as various iommu implementations also have limits (although
> usually much larger ones).

I though about exposing it through DMA-API, but didn't go that route as
I didn't want to extend a generic API for some SWIOTLB specifics. But if
there are more implementations that have a size limitation it makes
certainly sense to put it into the DMA-API. I'll change that in the
next version.

Thanks,

	Joerg