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arm64: mm: Offset TTBR1 to allow 52-bit PTRS_PER_PGD

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Enabling 52-bit VAs on arm64 requires that the PGD table expands from 64
entries (for the 48-bit case) to 1024 entries. This quantity,
PTRS_PER_PGD is used as follows to compute which PGD entry corresponds
to a given virtual address, addr:

pgd_index(addr) -> (addr >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1)

Userspace addresses are prefixed by 0's, so for a 48-bit userspace
address, uva, the following is true:
(uva >> PGDIR_SHIFT) & (1024 - 1) == (uva >> PGDIR_SHIFT) & (64 - 1)

In other words, a 48-bit userspace address will have the same pgd_index
when using PTRS_PER_PGD = 64 and 1024.

Kernel addresses are prefixed by 1's so, given a 48-bit kernel address,
kva, we have the following inequality:
(kva >> PGDIR_SHIFT) & (1024 - 1) != (kva >> PGDIR_SHIFT) & (64 - 1)

In other words a 48-bit kernel virtual address will have a different
pgd_index when using PTRS_PER_PGD = 64 and 1024.

If, however, we note that:
kva = 0xFFFF << 48 + lower (where lower[63:48] == 0b)
and, PGDIR_SHIFT = 42 (as we are dealing with 64KB PAGE_SIZE)

We can consider:
(kva >> PGDIR_SHIFT) & (1024 - 1) - (kva >> PGDIR_SHIFT) & (64 - 1)
 = (0xFFFF << 6) & 0x3FF - (0xFFFF << 6) & 0x3F	// "lower" cancels out
 = 0x3C0

In other words, one can switch PTRS_PER_PGD to the 52-bit value globally
provided that they increment ttbr1_el1 by 0x3C0 * 8 = 0x1E00 bytes when
running with 48-bit kernel VAs (TCR_EL1.T1SZ = 16).

For kernel configuration where 52-bit userspace VAs are possible, this
patch offsets ttbr1_el1 and sets PTRS_PER_PGD corresponding to the
52-bit value.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Steve Capper <steve.capper@arm.com>
[will: added comment to TTBR1_BADDR_4852_OFFSET calculation]
Signed-off-by: Will Deacon <will.deacon@arm.com>
This commit is contained in:
Steve Capper
2018-12-06 22:50:39 +00:00
committed by Will Deacon
parent e5d9915745
commit e842dfb5a2
5 changed files with 39 additions and 0 deletions
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23
arch/arm64/include/asm/assembler.h
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@@ -543,6 +543,29 @@ USER(\label, ic ivau, \tmp2) // invalidate I line PoU
mrs \rd, sp_el0
.endm
/*
* Offset ttbr1 to allow for 48-bit kernel VAs set with 52-bit PTRS_PER_PGD.
* orr is used as it can cover the immediate value (and is idempotent).
* In future this may be nop'ed out when dealing with 52-bit kernel VAs.
* ttbr: Value of ttbr to set, modified.
*/
.macro offset_ttbr1, ttbr
#ifdef CONFIG_ARM64_52BIT_VA
orr \ttbr, \ttbr, #TTBR1_BADDR_4852_OFFSET
#endif
.endm
/*
* Perform the reverse of offset_ttbr1.
* bic is used as it can cover the immediate value and, in future, won't need
* to be nop'ed out when dealing with 52-bit kernel VAs.
*/
.macro restore_ttbr1, ttbr
#ifdef CONFIG_ARM64_52BIT_VA
bic \ttbr, \ttbr, #TTBR1_BADDR_4852_OFFSET
#endif
.endm
/*
* Arrange a physical address in a TTBR register, taking care of 52-bit
* addresses.