Re: [PATCH] ppc32 copy_to_user dcbt fixup

[Danjel McGougan]

Bryan Rittmeyer wrote:
> copy_tofrom_user and copy_page use dcbt to prefetch source data [1].
> Since at least 2.4.17, these functions have been prefetching
> beyond the end of the source buffer, leading to two problems:
>
> 1. Subtly broken software cache coherency. If the area following src
> was invalidate_dcache_range'd prior to submitting for DMA,
> an out-of-bounds dcbt from copy_to_user of a separate slab object
> may read in the area before DMA completion. When the DMA does complete,
> data will not be loaded from RAM because stale data is already in cache.
> Thus you get a corrupt network packet, bogus audio capture, etc.
[snip]

I am no expert on the ppc arch, but in my humble opinion it seems 
strange to invalidate the dcache *before* the memory-writing 
DMA-transaction. The obvious solution is to invalidate the dcache 
*after* DMA completion. It seems hard to guarantee that nobody will 
touch the memory area in question during the DMA.

Just some clue-less comments from a linux-kernel lurker.

Regards,
Danjel
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