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New .nops directive, to aid Linux alternatives patching?

H.J. Lu hjl.tools@gmail.com
Thu Feb 8 20:28:00 GMT 2018 
On Thu, Feb 8, 2018 at 12:27 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Thu, Feb 8, 2018 at 12:18 PM, Andrew Cooper
> <andrew.cooper3@citrix.com> wrote:
>> On 08/02/2018 20:10, H.J. Lu wrote:
>>> On Thu, Feb 8, 2018 at 11:26 AM, Andrew Cooper
>>> <andrew.cooper3@citrix.com> wrote:
>>>> Hello,
>>>>
>>>> I realise this is a little bit niche, but how feasible would it be to
>>>> introduce a new .nops directive which takes a size parameter, and
>>>> outputs long nops covering the number of specified bytes?
>>>>
>>> Sounds to me you want a pseudo NOP instruction:
>>>
>>> pseudo-NOP N
>>>
>>> which generates a long NOP with N byte.  Is that correct.  If yes,
>>> what is the range of N?
>>
>> Currently 255 based on other implementation limits, and I expect that
>> ought to be long enough for anyone.  There is one existing user for
>> N=43, and I expect that to grow a bit.
>>
>> The real answer properly depends at what point it is more efficient to
>> jmp rather than wasting decode bandwidth decoding nops, and I don't know
>> the answer, but expect that it isn't larger than 255.
>>
>
> How about
>
> {nop} N
>
> If N is less than 15 bytes, it generates a long nop.   Otherwise, we use a jump
> instruction over nops.  Does it work for you?

N will be limited to 255.



-- 
H.J.
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