Re: [PATCH v2] livepatch: Add some basic LivePatch documentation

[Miroslav Benes]

On Tue, 26 Apr 2016, Balbir Singh wrote:

> > +  + Anything inlined into __schedule() can not be patched.
> > +
> > +    The switch_to macro is inlined into __schedule(). It switches the
> > +    context between two processes in the middle of the macro. It does
> > +    not save RIP in x86_64 version (contrary to 32-bit version). Instead,
> > +    the currently used __schedule()/switch_to() handles both processes.
> > +
> > +    Now, let's have two different tasks. One calls the original
> > +    __schedule(), its registers are stored in a defined order and it
> > +    goes to sleep in the switch_to macro and some other task is restored
> > +    using the original __schedule(). Then there is the second task which
> > +    calls patched__schedule(), it goes to sleep there and the first task
> > +    is picked by the patched__schedule(). Its RSP is restored and now
> > +    the registers should be restored as well. But the order is different
> > +    in the new patched__schedule(), so...
> > +
> > +    There is a work in progress to remove this limitation.
> > +
> 
> I am afraid the example requires more clarification. I don't quite get the order is different

Different order is not inevitable but perfectly possible (even probable). 
GCC may simply generate different object code for patched__schedule() than 
it did for __schedule(). The problem is when the prologue and epilogue are 
different.

Miroslav