linux-kernel - Re: [PATCH v4 2/7] iommu/core: split mapping to page sizes as supported by the hardware

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Message-ID: <4EBC3E20.20301@codeaurora.org>
Date:	Thu, 10 Nov 2011 13:12:00 -0800
From:	Stepan Moskovchenko <stepanm@...eaurora.org>
To:	Joerg Roedel <Joerg.Roedel@....com>
CC:	David Woodhouse <dwmw2@...radead.org>,
	Kai Huang <mail.kai.huang@...il.com>,
	Ohad Ben-Cohen <ohad@...ery.com>,
	iommu@...ts.linux-foundation.org, linux-omap@...r.kernel.org,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	linux-arm-kernel@...ts.infradead.org,
	David Brown <davidb@...eaurora.org>,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	Hiroshi Doyu <hdoyu@...dia.com>,
	KyongHo Cho <pullip.cho@...sung.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v4 2/7] iommu/core: split mapping to page sizes as supported
 by the hardware

On 11/10/2011 9:09 AM, Joerg Roedel wrote:
> The plan is to have a single DMA-API implementation for all IOMMU 
> drivers (X86 and ARM) which just uses the IOMMU-API. But to make this 
> performing reasonalbly well a few changes to the IOMMU-API are 
> required. I already have some ideas which we can discuss if you want.

I have been experimenting with an iommu_map_range call, which maps a 
given scatterlist of discontiguous physical pages into a contiguous 
virtual region at a given IOVA. This has some performance advantages 
over just calling iommu_map iteratively. First, it reduces the amount of 
table walking / calculation needed for mapping each page, given how you 
know that all the pages will be mapped into a single 
virtually-contiguous region (so in most cases, the first-level table 
calculation can be reused). Second, it allows one to defer the TLB (and 
sometimes cache) maintenance operations until the entire scatterlist has 
been mapped, rather than doing a TLB invalidate after mapping each page, 
as would have been the case if iommu_map were just being called from 
within a loop. Granted, just using iommu_map many times may be 
acceptable on the slow path, but I have seen significant performance 
gains when using this approach on the fast path.

Steve

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