Re: [Lightning] Code patching, context switching

[Paolo Bonzini]

> I want to use Lightning for user-level context switching, similar to
> GNU Pth.

Why not use makecontext/swapcontext?  (which is what Pth uses except for 
signal handling).

>  1. There is currently no Lightning equivalent to the x86's 'pusha'
> instruction [1], correct?
>  2. I assume then that I would have to save all registers by iterating
> over JIT_R/V/FPR manually while generating this code. Are there any
> registers or other state that aren't accessible via Lightning that
> might impact context switching?

Yes.  On the PowerPC, the argument registers are *not* available to the 
user, because they are caller-save while GNU lightning always provides 
the illusion that they are callee-save.

> /* from and to are buffers sufficiently large to hold the register file */
> static void ctxt_switch(char *from, char *to) {
>   from = from;
>   to = to;
>   from = from;
>   to = to;
>   ...
> }
>
> I can then take the address of ctxt_switch and pass it to Lightning as
> the code buffer and patch the contents of ctxt_switch to perform an
> actual context switch. The other alternative is to simply patch in a
> direct jump at &ctxt_switch into my code generated elsewhere. Any
> other thoughts or recommendations?

I think that the code is readonly on all the platforms I ever looked at 
that have an MMU.  You would have to use mprotect first; then I think 
that patching a direct jump is fast enough.

However, beware the compiler.  It will remove all those pseudo-NOP 
statements that (it looks like this, at least) you placed to make 
ctxt_switch big enough.  It might also inline ctxt_switch, which would 
screw things up a lot.  I would just make ctxt_switch a function pointer.

Paolo