diff options
| author | Russell King <rmk+kernel@arm.linux.org.uk> | 2014-03-25 19:45:31 +0000 |
|---|---|---|
| committer | Russell King <rmk+kernel@arm.linux.org.uk> | 2014-04-03 22:46:34 +0100 |
| commit | e26a9e00afc482b971afcaef1db8c9034d4d6d7c (patch) | |
| tree | 6ed907fccdafda9a6656794db0543692f7e5d46f /arch/arm/kernel/head.S | |
| parent | 83b3f64d46d1d9bf1e0be1d16f805ccdd52d31c6 (diff) | |
ARM: Better virt_to_page() handling
virt_to_page() is incredibly inefficient when virt-to-phys patching is
enabled. This is because we end up with this calculation:
page = &mem_map[asm virt_to_phys(addr) >> 12 - __pv_phys_offset >> 12]
in assembly. The asm virt_to_phys() is equivalent this this operation:
addr - PAGE_OFFSET + __pv_phys_offset
and we can see that because this is assembly, the compiler has no chance
to optimise some of that away. This should reduce down to:
page = &mem_map[(addr - PAGE_OFFSET) >> 12]
for the common cases. Permit the compiler to make this optimisation by
giving it more of the information it needs - do this by providing a
virt_to_pfn() macro.
Another issue which makes this more complex is that __pv_phys_offset is
a 64-bit type on all platforms. This is needlessly wasteful - if we
store the physical offset as a PFN, we can save a lot of work having
to deal with 64-bit values, which sometimes ends up producing incredibly
horrid code:
a4c: e3009000 movw r9, #0
a4c: R_ARM_MOVW_ABS_NC __pv_phys_offset
a50: e3409000 movt r9, #0 ; r9 = &__pv_phys_offset
a50: R_ARM_MOVT_ABS __pv_phys_offset
a54: e3002000 movw r2, #0
a54: R_ARM_MOVW_ABS_NC __pv_phys_offset
a58: e3402000 movt r2, #0 ; r2 = &__pv_phys_offset
a58: R_ARM_MOVT_ABS __pv_phys_offset
a5c: e5999004 ldr r9, [r9, #4] ; r9 = high word of __pv_phys_offset
a60: e3001000 movw r1, #0
a60: R_ARM_MOVW_ABS_NC mem_map
a64: e592c000 ldr ip, [r2] ; ip = low word of __pv_phys_offset
Reviewed-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Diffstat (limited to 'arch/arm/kernel/head.S')
| -rw-r--r-- | arch/arm/kernel/head.S | 17 |
1 files changed, 9 insertions, 8 deletions
diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S index 914616e0bdcd..3aca959fee8d 100644 --- a/arch/arm/kernel/head.S +++ b/arch/arm/kernel/head.S @@ -584,9 +584,10 @@ __fixup_pv_table: subs r3, r0, r3 @ PHYS_OFFSET - PAGE_OFFSET add r4, r4, r3 @ adjust table start address add r5, r5, r3 @ adjust table end address - add r6, r6, r3 @ adjust __pv_phys_offset address + add r6, r6, r3 @ adjust __pv_phys_pfn_offset address add r7, r7, r3 @ adjust __pv_offset address - str r8, [r6, #LOW_OFFSET] @ save computed PHYS_OFFSET to __pv_phys_offset + mov r0, r8, lsr #12 @ convert to PFN + str r0, [r6, #LOW_OFFSET] @ save computed PHYS_OFFSET to __pv_phys_pfn_offset strcc ip, [r7, #HIGH_OFFSET] @ save to __pv_offset high bits mov r6, r3, lsr #24 @ constant for add/sub instructions teq r3, r6, lsl #24 @ must be 16MiB aligned @@ -600,7 +601,7 @@ ENDPROC(__fixup_pv_table) 1: .long . .long __pv_table_begin .long __pv_table_end -2: .long __pv_phys_offset +2: .long __pv_phys_pfn_offset .long __pv_offset .text @@ -688,11 +689,11 @@ ENTRY(fixup_pv_table) ENDPROC(fixup_pv_table) .data - .globl __pv_phys_offset - .type __pv_phys_offset, %object -__pv_phys_offset: - .quad 0 - .size __pv_phys_offset, . -__pv_phys_offset + .globl __pv_phys_pfn_offset + .type __pv_phys_pfn_offset, %object +__pv_phys_pfn_offset: + .word 0 + .size __pv_phys_pfn_offset, . -__pv_phys_pfn_offset .globl __pv_offset .type __pv_offset, %object |