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SubjectRe: [PATCH v3 0/6] Static calls
On Fri, Jan 11, 2019 at 03:48:59PM +0000, Nadav Amit wrote:
> > I liked the idea, BUT, how would it work for callee-saved PV ops? In
> > that case there's only one clobbered register to work with (rax).
>
> That’s would be more tricky. How about using a per-CPU trampoline code to
> hold a direct call to the target and temporarily disable preemption (which
> might be simpler by disabling IRQs):
>
> Static-call modifier:
>
> 1. synchronize_sched() to ensure per-cpu trampoline is not used
> 2. Patches the jmp in a per-cpu trampoline (see below)
> 3. Saves the call source RIP in [per-cpu scratchpad RIP] (below)
> 4. Configures the int3 handler to use static-call int3 handler
> 5. Patches the call target (as it currently does).
>
> Static-call int3 handler:
> 1. Changes flags on the stack to keep IRQs disabled on return
> 2. Jumps to per-cpu trampoline on return
>
> Per-cpu trampoline:
> push [per-CPU scratchpad RIP]
> sti
> jmp [ target ] (this one is patched)
>
> Note that no IRQ should be possible between the STI and the JMP due to STI
> blocking.
>
> What do you say?

This could work, but it's more complex than I was hoping for.

My current leading contender is to do call emulation in the #BP handler,
either by making a gap or by doing Andy's longjmp-style thingie.

--
Josh

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