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arm64: mm: permit use of tagged pointers at EL0

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TCR.TBI0 can be used to cause hardware address translation to ignore the
top byte of userspace virtual addresses. Whilst not especially useful in
standard C programs, this can be used by JITs to `tag' pointers with
various pieces of metadata.

This patch enables this bit for AArch64 Linux, and adds a new file to
Documentation/arm64/ which describes some potential caveats when using
tagged virtual addresses.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
This commit is contained in:
Will Deacon
2013-06-12 16:28:04 +01:00
committed by Catalin Marinas
parent 909e3ee411
commit d50240a5f6
4 changed files with 37 additions and 1 deletions
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arch/arm64/include/asm/pgtable-hwdef.h
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@@ -122,5 +122,6 @@
#define TCR_TG1_64K (UL(1) << 30)
#define TCR_IPS_40BIT (UL(2) << 32)
#define TCR_ASID16 (UL(1) << 36)
#define TCR_TBI0 (UL(1) << 37)
#endif