Re: [PATCH v3 0/6] Static calls

[Nadav Amit]

> On Jan 11, 2019, at 1:22 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> 
> On Fri, Jan 11, 2019 at 12:46:39PM -0800, Linus Torvalds wrote:
>> On Fri, Jan 11, 2019 at 12:31 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>>> I was referring to the fact that a single static call key update will
>>> usually result in patching multiple call sites.  But you're right, it's
>>> only 1-2 trampolines per text_poke_bp() invocation.  Though eventually
>>> we may want to batch all the writes like what Daniel has proposed for
>>> jump labels, to reduce IPIs.
>> 
>> Yeah, my suggestion doesn't allow for batching, since it would
>> basically generate one trampoline for every rewritten instruction.
> 
> As Andy said, I think batching would still be possible, it's just that
> we'd have to create multiple trampolines at a time.
> 
> Or... we could do a hybrid approach: create a single custom trampoline
> which has the call destination patched in, but put the return address in
> %rax -- which is always clobbered, even for callee-saved PV ops.  Like:
> 
> trampoline:
> 	push %rax
> 	call patched-dest
> 
> That way the batching could be done with a single trampoline
> (particularly if using rcu-sched to avoid the sti hack).

I donât see RCU-sched solves the problem if you donât disable preemption. On
a fully preemptable kernel, you can get preempted between the push and the
call (jmp) or before the push. RCU-sched can then finish, and the preempted
task may later jump to a wrong patched-dest.