Ralf Baechle wrote:
> Cache aliases. When the same page of physical memory is mapped twice to
> user space, let's say at address addr and addr + PAGE_SIZE this is normally
> harmless although wasteful on processors with virtually indexed caches as
> long as the page is mapped read-only such as in case of ZERO_PAGE.
>
> If the same thing happens with a writable page there is the chance of
> severe data corruption. Some members of the R4000 family are now trying
> to be helpful by throwing the kernel a "virtual coherency" exception. The
> bad news about this exception is there might be thousands (the theoretical
> worst case would be millions) of it in a single second, so servicing can be
> very expensive. For the ZERO page this can be avoided by using several
> pages mapped in a way such that their addresses don't conflict.
Note that it can be a *very expensive* waste even on machines that do
this in hardware. Colouring the zeropage can have sizable performance
advantages for virtually no cost.
-hpa
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