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

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