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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>