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    SubjectRe: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management
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    Zach Pfeffer <zpfeffer@codeaurora.org> writes:

    > 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.

    How does this compare with the x86 pat code?

    >> 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.

    To my knowledge mismatch of mapping attributes is a problem on most
    cpus on every architecture. I don't see it making sense to encourage
    coding constructs that will fail in the strangest most difficult to
    debug ways.

    Eric



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