linux-kernel - Re: Question about iommu on x86

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Message-ID: <20110531134519.GC14641@dumpdata.com>
Date:	Tue, 31 May 2011 09:45:19 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Daniel Haid <d.haid@...i.tv>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: Question about iommu on x86_64 and radeon driver.

On Fri, May 27, 2011 at 11:20:09PM +0100, Daniel Haid wrote:
> >The DRM code is not used anymore. It was used by XFree86 but nowadays
> >the TTM code is used instead.
> 
> I see
> 
> >Ah, this would be a lengthy writeup.. so did you look at the
> >link I pointed you to?
> 
> Yes I have started, but do not understand everything yet.
> 
> What are MFNs? Is this something Xen-specific?
>

<nods>
> 
> An does your post imply that on bare metal with "swiotlb=force
> iommu=soft"
> alloc_page with GFP_DMA32 does also not allocate under 4GB?

Noo.. It does, but the normal assumption of 'phys_to_virt' == 'phys_to_bus' is
not valid anymore. Think of a buffer (swiotlb) which has a pool
of pages and when a PCI device wants a page, it hands one out. It also has
other functionality such as 'mapping' of an already allocated page. If the
PCI device asks the IOMMU (swiotlb) to map a page (and if you have 'swiotlb=force'
the page provided has been allocated above 4GB and the device can only handle
up to 32-bit), then swiotlb gives out a page from its own pool. You now have
two addresses: the one from the PCI pool (swiotlb) and the one you already
allocated. You are suppose to program your PCI card to read/write data to the
page provided from the IOMMU (so the swiotlb), which means that it won't
write to the page you had allocated. Hence there are a calls, such as 'sync_page'..
which will copy the contents from the swiotlb page to the one you had allocated
(or vice-versa). This is called 'bounce buffer'.

The radeon (and nouveau) don't have the code to call the 'sync_page', and
you wouldn't really want to do so - as it slows down the performance of the
machine. There exists another mechanism which is to allocate the pages
at the start, and not do mapping later one.

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