Re: Caches, page coloring, virtual indexed caches, and more

[Jamie Lokier <lk@tantalophile.demon.co.uk>]

Ralf Baechle wrote:
> > mremap.  Linux specific but pretty much the same as mmap, but easier.
> > We just enforce that the virtual address of the source of mremap,
> > and the destination of mremap match on VIRT_INDEX_BITS.
> 
> Correct and as mremap doesn't take any address argument we won't break
> any expecations on the properties of the returned address in mmap.

See MREMAP_FIXED.  There is an address argument, not mentioned in the
manpage (man-pages 1.30).

> > Hmm.  This doesn't sound right.  And this sounds like a silly way to
> > use reverse mappings anyway, since you can do it up front in mmap and
> > their kin.  Which means you don't have to slow any of the page fault
> > logic up.
> 
> Then how do you handle something like:
> 
>   fd = open(TESTFILE, O_RDWR | O_CREAT, 664);
>   res = write(fd, one, 4096);
>   mmap(addr            , PAGE_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
>   mmap(addr + PAGE_SIZE, PAGE_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
> 
> If both mappings are immediately created accessible you'll directly endup
> with aliases.  There is no choice, if the pagesize is only 4kb an R4x00
> will create aliases in the case.  Bad.

Indeed, a particularly nice way to handle circular buffers for DSP
algorithms provided it works :-)

-- Jamie
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