Re: [RFC][PATCH 0/9] Variable size jump_label support

[Steven Rostedt]

On Mon, 7 Oct 2019 13:26:06 +0200
Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> [ Sorry, fixed the Cc:lkml line. ]

/me joining the fun.

> 
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > These here patches are something I've been poking at for a while, 
> > enabling jump_label to use 2 byte jumps/nops.
> > 
> > It _almost_ works :-/
> > 
> > That is, you can build some kernels with it (x86_64-defconfig for 
> > example works just fine).
> > 
> > The problem comes when GCC generates a branch into another section, 
> > mostly .text.unlikely. At that point GAS just gives up and throws a fit 
> > (more details in the last patch).
> > 
> > Aside from anyone coming up with a really clever GAS trick, I don't see 
> > how we can do this other than:  
> 
> >  - use 'jmp' and get objtool to rewrite the text. Steven has earlier proposed
> >    something like that (using recordmcount) and Linus hated that.  
> 
> As long as GCC+GAS correctly generates a 2-byte or 5-byte JMP depending 
> on the target distance, the objtool solution should work fine, shouldn't 
> it?
> 
> I can see the recordmcount solution sucking, it would depend on early 
> kernel patchery. But build time patchery is something we already depend 
> on, so assuming some objtool catastrophy it's a more robust solution, 
> isn't it?
> 

Actually, even back then I said that it would be best to merge all the
tools into one (I just didn't have the time to implement it), and then
we could pull this off. I have one of my developers working to merge
record-mcount into objtool now (there's been some patches floating
around).

Then with a single tool, it shouldn't be controversial.

-- Steve