Messages in this thread | | | Date | Sun, 18 Jul 2010 23:52:33 -0700 | From | Zach Pfeffer <> | Subject | Re: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management |
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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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