Zach Pfeffer <zpfeffer@xxxxxxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
