Re: Why is text_mutex used in jump_label_transform for x86_64

["chengjian (D)" <cj.chengjian@huawei.com>]

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