Re: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management

[Zach Pfeffer <zpfeffer@codeaurora.org>]

On Thu, Jul 15, 2010 at 09:55:35AM +0100, Russell King - ARM Linux wrote:
> On Wed, Jul 14, 2010 at 06:29:58PM -0700, Zach Pfeffer wrote:
> > The VCM ensures that all mappings that map a given physical buffer:
> > IOMMU mappings, CPU mappings and one-to-one device mappings all map
> > that buffer using the same (or compatible) attributes. At this point
> > the only attribute that users can pass is CACHED. In the absence of
> > CACHED all accesses go straight through to the physical memory.
> 
> So what you're saying is that if I have a buffer in kernel space
> which I already have its virtual address, I can pass this to VCM and
> tell it !CACHED, and it'll setup another mapping which is not cached
> for me?

Not quite. The existing mapping will be represented by a reservation
from the prebuilt VCM of the VM. This reservation has been marked
non-cached. Another reservation on a IOMMU VCM, also marked non-cached
will be backed with the same physical memory. This is legal in ARM,
allowing the vcm_back call to succeed. If you instead passed cached on
the second mapping, the first mapping would be non-cached and the
second would be cached. If the underlying architecture supported this
than the vcm_back would go through.

> 
> You are aware that multiple V:P mappings for the same physical page
> with different attributes are being outlawed with ARMv6 and ARMv7
> due to speculative prefetching.  The cache can be searched even for
> a mapping specified as 'normal, uncached' and you can get cache hits
> because the data has been speculatively loaded through a separate
> cached mapping of the same physical page.

I didn't know that. Thanks for the heads up.

> FYI, during the next merge window, I will be pushing a patch which makes
> ioremap() of system RAM fail, which should be the last core code creator
> of mappings with different memory types.  This behaviour has been outlawed
> (as unpredictable) in the architecture specification and does cause
> problems on some CPUs.

That's fair enough, but it seems like it should only be outlawed for
those processors on which it breaks.

> 
> We've also the issue of multiple mappings with differing cache attributes
> which needs addressing too...

The VCM has been architected to handle these things.

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