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SubjectRe: How to efficiently handle DMA and cache on ARMv7 ? (was "Is get_user_pages() enough to prevent pages from being swapped out ?")
Russell King - ARM Linux wrote:
> On Fri, Aug 07, 2009 at 09:58:30AM +0200, Laurent Pinchart wrote:
> > Sorry about this, but I'm not sure to understand the speculative prefetching
> > cache issue completely.
>
> The general case with speculative prefetching is that if memory is
> accessible, it can be prefetched.
>
> In other words, if we mapped devices without NX (non-exec) set, the
> CPU can prefetch instructions from devices, causing random read
> accesses. Yes, I know it sounds crazy, but that's what I'm told
> _can_ happen.

1. Does the architecture not prevent speculative instruction
prefetches from crossing a page boundary? It would be handy under the
circumstances.

2. Is NX available on all the CPUs with speculative prefetching
behaviour? If it is, just use that for device mappings?

-- Jamie


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