Messages in this thread | | | Date | Thu, 07 Jun 2007 11:04:04 -0700 | From | "H. Peter Anvin" <> | Subject | Re: why does the macro "ZERO_PAGE" take an argument? |
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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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