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SubjectRe: Why is text_mutex used in jump_label_transform for x86_64

On 2020/4/6 22:10, Will Deacon wrote:
> On Mon, Apr 06, 2020 at 11:15:51AM +0200, Peter Zijlstra wrote:
>> On Mon, Apr 06, 2020 at 04:39:11PM +0800, chengjian (D) wrote:
>>> On 2020/3/20 18:27, Peter Zijlstra wrote:
>>>> It depends on the architecture details of how self-modifying code works.
>>>> In particular, x86 is a variable instruction length architecture and
>>>> needs extreme care -- it's implementation requires there only be a
>>>> single text modifier at any one time, hence the use of text_mutex.
>>>> ARM64 OTOH is, like most RISC based architectures, a fixed width
>>>> instruction architecture. And in particular it can re-write certain
>>>> (branch) instructions with impunity (see their
>>>> aarch64_insn_patch_text_nosync()). Which is why they don't need
>>>> additional serialization.
>>> Hi, Peter
>>> Thank you very much for your reply.
>>> X86 is a variable-length instruction, only one byte modification of the
>>> instruction
>>> can be regarded as atomic. so we must be very careful when modifying
>>> instructions
>>> concurrently.
>> Close enough.
>>> For other architectures such as ARM64, the modification of some instructions
>>> can be
>>> considered atomic, (Eg. nop -> jmp/b). The set of instructions that can be
>>> executed
>>> by one thread of execution as they are being modified by another thread of
>>> execution
>>> without requiring explicit synchronization.
>>> In ARM64 Architecture Reference Manual, I find that:
>>>     Concurrent modification and execution of instructions can lead to the
>>> resulting instruction performing any behavior
>>>     that can be achieved by executing any sequence of instructions that can
>>> be executed from the same Exception level,
>>>     except where each of the instruction before modification and the
>>> instruction after modification is one of a B, BL, BRK,
>>>     HVC, ISB, NOP, SMC, or SVC instruction.
>>>     For the B, BL, BRK, HVC, ISB, NOP, SMC, and SVC instructions the
>>> architecture guarantees that, after modification of the
>>>     instruction, behavior is consistent with execution of either:
>>>     • The instruction originally fetched.
>>>     • A fetch of the modified instruction
>>> So we can safely modify jump_label for ARM64(from NOP to b or form b to
>>> NOP).
>>> Is my understanding correct?
>> I think so; but I'm really not much of an ARM64 person. FWIW I think I
>> remember Will saying the same is true of ARM (32bit) and they could
>> implement the same optimization, but so far nobody has bothered doing
>> so. But please, ask an ARM64 maintainer and don't take my word for this.
> On 32-bit there are complications with Thumb-2 instructions where you can
> have a mixture of 16-bit and 32-bit encodings, so you have to be pretty
> careful there.
> For arm64, we have aarch64_insn_patch_text_nosync() which we use to toggle
> jump labels.
> Will
> .

Hi, Peter and Will

    I have learned.

    I truly appreciate your timely help.

    Thanks a lot.

    -- Cheng Jian

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