linux-kernel - Re: NVMe vs DMA addressing limitations

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Message-ID: <9020459.Ga31IGQ4TP@wuerfel>
Date:   Tue, 10 Jan 2017 16:02:28 +0100
From:   Arnd Bergmann <arnd@...aro.org>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     Christoph Hellwig <hch@....de>,
        Nikita Yushchenko <nikita.yoush@...entembedded.com>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>, Jens Axboe <axboe@...com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-renesas-soc@...r.kernel.org,
        Simon Horman <horms@...ge.net.au>, linux-pci@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        artemi.ivanov@...entembedded.com
Subject: Re: NVMe vs DMA addressing limitations

On Tuesday, January 10, 2017 3:48:39 PM CET Christoph Hellwig wrote:
> On Tue, Jan 10, 2017 at 12:01:05PM +0100, Arnd Bergmann wrote:
> > Another workaround me might need is to limit amount of concurrent DMA
> > in the NVMe driver based on some platform quirk. The way that NVMe works,
> > it can have very large amounts of data that is concurrently mapped into
> > the device.
> 
> That's not really just NVMe - other storage and network controllers also
> can DMA map giant amounts of memory.  There are a couple aspects to it:
> 
>  - dma coherent memoery - right now NVMe doesn't use too much of it,
>    but upcoming low-end NVMe controllers will soon start to require
>    fairl large amounts of it for the host memory buffer feature that
>    allows for DRAM-less controller designs.  As an interesting quirk
>    that is memory only used by the PCIe devices, and never accessed
>    by the Linux host at all.

Right, that is going to become interesting, as some platforms are
very limited with their coherent allocations.

>  - size vs number of the dynamic mapping.  We probably want the dma_ops
>    specify a maximum mapping size for a given device.  As long as we
>    can make progress with a few mappings swiotlb / the iommu can just
>    fail mapping and the driver will propagate that to the block layer
>    that throttles I/O.

Good idea.

	Arnd