New .nops directive, to aid Linux alternatives patching?

[H.J. Lu]

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?


-- 
H.J.