New .nops directive, to aid Linux alternatives patching?

[Andrew Cooper]

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.

~Andrew