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Re: Cache aliasing on >32k VIPT caches
On Aug 25, 2008, at 5:54 AM, Imre Deak wrote:
according to the ARMv6 TRM:
1. If multiple Virtual Addresses are mapped onto the same physical
address then for
all mappings, bits [13:12] of the Virtual Addresses must be
equal and the same as
bits [13:12] of the physical address. Imposing this requirement
on the virtual
addresses is sometimes called page coloring.
The same physical address can be mapped by TLB entries of
different page sizes,
including page sizes over 4KB.
This is used when mapping large pages and sections by mem(4). I
support to do that.
2. Alternatively, if all mappings to a physical address are of a
page size equal to
4KB, then the restriction that bits[13:12] of the Virtual
Address must equal
bits[13:12] of the physical address is not necessary. Bits
[13:12] of all Virtual
Address aliases must still be equal.
So based on the above at the least we should adhere to 2., I'm
right now there isn't any case where we need 1. The current pmap
code only considers
bit 12 to distinguish between colors, does this need to be
corrected? As far as I
understand only bit 12 is significant for picking the cache index
and thus color,
which would make the current code correct, but then again the TRM
might just know
__BIT(9 + CPU_CT_xSIZE_SIZE(isize)
The code as it exists is correct.
For 16KB/32KB/64KB, CPU_CT_xSIZE_SIZE(isize) is 0b101 (5) / 0b110
(6) / 0b111 (6).
CPU_CT_xSIZE_ASSOC(size) is always 0b010 (2).
So the __BIT() expression is __BIT(12), __BIT(13), __BIT(14),
Which gives a mask of 0, PAGE_SIZE, 3*PAGE_SIZE respectively.
For both 16KB and 64KB we use the right value, so why doesn't 32KB use
3*PAGE_SIZE as well? It has to do with cache organization...
Each cache line is 32 bytes and we have 4 ways, so that gives us 128
16KB. Note that 16KB doesn't need page coloring so it must be able
address to its unique (set, way).
Now 64KB needs 512 sets or 4x128 sets. The page color is used to
which of the four 128-entry sets to place the cache entry. That's
needs to be matched.
But 32KB only has 256 or 2x128 sets. So there are only two
the page color. So while we could use 4 page colors, there is no
the upper bit will be ignored. That will just result in additional
flushing to no real effect.
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