linux-kernel - Re: [PATCH v2 4/5] iommu: Use dma_addr

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Date:	Fri, 9 May 2014 09:32:26 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Arnd Bergmann <arnd@...db.de>, linux-doc@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Joerg Roedel <joro@...tes.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Liviu Dudau <Liviu.Dudau@....com>,
	linux-kernel@...r.kernel.org,
	James Bottomley <jbottomley@...allels.com>,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH v2 4/5] iommu: Use dma_addr_t for IOVA arguments

On Fri, May 09, 2014 at 10:58:50AM +0100, David Woodhouse wrote:
> On Thu, 2014-05-08 at 14:30 -0600, Bjorn Helgaas wrote:
> > I doubt there would be a noticeable performance effect since these are
> > relatively low-frequency interfaces (map, unmap, report_fault), 
> 
> That point of view makes me sad.

Don't be sad; Arnd successfully fended off the challenge :)

> There are people who care deeply about the performance of IOMMU API
> map/unmap. It isn't used *just* for virtual machines any more. See
> drivers/infiniband/hw/usnic/usnic_uiom.c for example.

Of course we should care about IOMMU API performance.  We should also
care about interface consistency, and it seems there's a tradeoff in
this case.  I said "relatively" because I expect map/unmap to be less
frequent than read/write operations that use the mapping.  I don't
know anything about infiniband, so maybe that assumption is false
there.

> I also hold out *some* hope for consolidating the map/unmap functions
> for the IOMMU and DMA APIs at some point. The main difference is that
> the DMA API allocates an IOVA for itself, while the IOMMU API is given
> the bus address too.

I find this aspect of these APIs confusing, so I agree that it would
be nice if these could be consolidated somehow.

Bjorn
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