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From: "hch@lst.de" <hch@lst.de>
To: Robin Murphy <robin.murphy@arm.com>
Cc: "davidm@hpl.hp.com" <davidm@hpl.hp.com>,
	"ralf@oss.sgi.com" <ralf@oss.sgi.com>,
	Linuxarm <linuxarm@huawei.com>,
	"linux@armlinux.org.uk" <linux@armlinux.org.uk>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"sailer@ife.ee.ethz.ch" <sailer@ife.ee.ethz.ch>,
	"Jay.Estabrook@compaq.com" <Jay.Estabrook@compaq.com>,
	"dagum@barrel.engr.sgi.com" <dagum@barrel.engr.sgi.com>,
	"andrea@suse.de" <andrea@suse.de>,
	"grundler@cup.hp.com" <grundler@cup.hp.com>,
	"jens.axboe@oracle.com" <jens.axboe@oracle.com>,
	"hch@lst.de" <hch@lst.de>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: Constantly map and unmap of streaming DMA buffers with IOMMU backend might cause serious performance problem
Date: Fri, 15 May 2020 16:45:22 +0200	[thread overview]
Message-ID: <20200515144522.GA25652@lst.de> (raw)
In-Reply-To: <36d67d68-4381-c7a7-dcf1-6383bd9ae0ad@arm.com>

On Fri, May 15, 2020 at 01:10:21PM +0100, Robin Murphy wrote:
>> Meanwhile, for the safety of buffers, lower-layer drivers need to make certain the buffers have already been unmapped in iommu before those buffers go back to buddy for other users.
>
> That sounds like it would only have benefit in a very small set of specific 
> circumstances, and would be very difficult to generalise to buffers that 
> are mapped via dma_map_page() or dma_map_single(). Furthermore, a 
> high-level API that affects a low-level driver's interpretation of 
> mid-layer API calls without the mid-layer's knowledge sounds like a hideous 
> abomination of anti-design. If a mid-layer API lends itself to inefficiency 
> at the lower level, it would seem a lot cleaner and more robust to extend 
> *that* API for stateful buffer reuse. Failing that, it might possibly be 
> appropriate to approach this at the driver level - many of the cleverer 
> network drivers already implement buffer pools to recycle mapped SKBs 
> internally, couldn't the "zip driver" simply try doing something like that 
> for itself?

Exactly.  If you upper consumer of the DMA API keeps reusing the same
pages just map them once and use dma_sync_* to transfer ownership as
needed.
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu

WARNING: multiple messages have this Message-ID
From: "hch@lst.de" <hch@lst.de>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Song Bao Hua <song.bao.hua@hisilicon.com>,
	"davidm@hpl.hp.com" <davidm@hpl.hp.com>,
	"ralf@oss.sgi.com" <ralf@oss.sgi.com>,
	Linuxarm <linuxarm@huawei.com>,
	"linux@armlinux.org.uk" <linux@armlinux.org.uk>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"sailer@ife.ee.ethz.ch" <sailer@ife.ee.ethz.ch>,
	"Jay.Estabrook@compaq.com" <Jay.Estabrook@compaq.com>,
	"dagum@barrel.engr.sgi.com" <dagum@barrel.engr.sgi.com>,
	"andrea@suse.de" <andrea@suse.de>,
	"grundler@cup.hp.com" <grundler@cup.hp.com>,
	"jens.axboe@oracle.com" <jens.axboe@oracle.com>,
	"hch@lst.de" <hch@lst.de>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"m.szyprowski@samsung.com" <m.szyprowski@samsung.com>
Subject: Re: Constantly map and unmap of streaming DMA buffers with IOMMU backend might cause serious performance problem
Date: Fri, 15 May 2020 16:45:22 +0200	[thread overview]
Message-ID: <20200515144522.GA25652@lst.de> (raw)
In-Reply-To: <36d67d68-4381-c7a7-dcf1-6383bd9ae0ad@arm.com>

On Fri, May 15, 2020 at 01:10:21PM +0100, Robin Murphy wrote:
>> Meanwhile, for the safety of buffers, lower-layer drivers need to make certain the buffers have already been unmapped in iommu before those buffers go back to buddy for other users.
>
> That sounds like it would only have benefit in a very small set of specific 
> circumstances, and would be very difficult to generalise to buffers that 
> are mapped via dma_map_page() or dma_map_single(). Furthermore, a 
> high-level API that affects a low-level driver's interpretation of 
> mid-layer API calls without the mid-layer's knowledge sounds like a hideous 
> abomination of anti-design. If a mid-layer API lends itself to inefficiency 
> at the lower level, it would seem a lot cleaner and more robust to extend 
> *that* API for stateful buffer reuse. Failing that, it might possibly be 
> appropriate to approach this at the driver level - many of the cleverer 
> network drivers already implement buffer pools to recycle mapped SKBs 
> internally, couldn't the "zip driver" simply try doing something like that 
> for itself?

Exactly.  If you upper consumer of the DMA API keeps reusing the same
pages just map them once and use dma_sync_* to transfer ownership as
needed.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  reply	other threads:[~2020-05-15 14:45 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-15  8:19 Constantly map and unmap of streaming DMA buffers with IOMMU backend might cause serious performance problem Song Bao Hua
2020-05-15  8:19 ` Song Bao Hua
2020-05-15 12:10 ` Robin Murphy
2020-05-15 12:10   ` Robin Murphy
2020-05-15 14:45   ` hch [this message]
2020-05-15 14:45     ` hch
2020-05-15 21:33     ` Song Bao Hua
2020-05-15 21:33       ` Song Bao Hua
2020-05-15 22:12       ` Robin Murphy
2020-05-15 22:12         ` Robin Murphy
2020-05-15 22:45   ` Song Bao Hua
2020-05-15 22:45     ` Song Bao Hua

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