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Date:   Fri, 11 Jan 2019 10:07:39 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Nadav Amit <namit@vmware.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [PATCH v3 0/6] Static calls
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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