Kernel version：2.6.14
This code uses the preload instruction pld (preload). The capital PLD is a macro, which is defined as follows (linux/include/asm-arm/assembler.h):
pld, that is, preload instruction, pld instruction is only valid in armv5 and above. Using pld instruction can prompt ARM to fill cache line in advance. The offset in pld instruction is very particular. It is usually a multiple of 64-byte.
Function: cache preload (PLD, PreLoad), use pld to instruct the storage system to read from the memory address specified by the following instructions. The storage system can use this method to speed up future memory access.
Format: pld[Rn,{offset}]
Where:
Reference address：Use of prefetch command pld in ARM
CPU architecture：ARM920T
Recently, I was analyzing copy_from_user of ARM Linux. The main function is implemented by __arch_copy_from_user. This function is defined in the linux/arch/arm/lib/uaccess.S file and is implemented using assembly. Some source codes are as follows:
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- ENTRY(__arch_copy_from_user)
- stmfd sp!, {r0, r2, r4 - r7, lr}
- cmp r2, #4
- blt .cfu_not_enough
- PLD( pld [r1, #0] ) //Because r0 and r1 will be used later, pre-read to speed up memory access
- PLD( pld [r0, #0] )
- ands ip, r0, #3
- bne .cfu_dest_not_aligned
- .cfu_dest_aligned:
- ands ip, r1, #3
- bne .cfu_src_not_aligned
- ...
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- #if __LINUX_ARM_ARCH__ >= 5
- #define PLD(code...) code
- #else
- #define PLD(code...)
- #endif
Function: cache preload (PLD, PreLoad), use pld to instruct the storage system to read from the memory address specified by the following instructions. The storage system can use this method to speed up future memory access.
Format: pld[Rn,{offset}]
Where:
- Rn The base address register of the memory.
- Offset The offset added to Rn.
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