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New .nops directive, to aid Linux alternatives patching?

Andrew Cooper andrew.cooper3@citrix.com
Thu Feb 8 20:33:00 GMT 2018 
On 08/02/2018 20:28, H.J. Lu wrote:
> 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.

Do you mean up to 255 bytes of adjacent long nops, or still a jump if
over 15 bytes?  For alternatives in the range of 15-30, a jmp is almost
certainly slower than executing through the nops.  The ORM isn't clear
where the split lies, and I expect it is very uarch specific.

~Andrew
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