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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andy Lutomirski <luto@kernel.org>
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Subject: Re: [patch V6 10/37] x86/entry: Switch XEN/PV hypercall entry to IDTENTRY
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Date:   Wed, 20 May 2020 16:13:26 +0200
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Andy Lutomirski <luto@kernel.org> writes:
> On Tue, May 19, 2020 at 11:58 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>> Which brings you into the situation that you call schedule() from the
>> point where we just moved it out. If we would go there we'd need to
>> ensure that RCU is watching as well. idtentry_exit() might have it
>> turned off ....
>
> I don't think this is possible.  Once you untangle all the wrappers,
> the call sites are effectively:
>
> __this_cpu_write(xen_in_preemptible_hcall, true);
> CALL_NOSPEC to the hypercall page
> __this_cpu_write(xen_in_preemptible_hcall, false);
>
> I think IF=1 when this happens, but I won't swear to it.  RCU had
> better be watching.
>
> As I understand it, the one and only situation Xen wants to handle is
> that an interrupt gets delivered during the hypercall.  The hypervisor
> is too clever for its own good and deals with this by rewinding RIP to
> the beginning of whatever instruction did the hypercall and delivers
> the interrupt, and we end up in this handler.  So, if this happens,
> the idea is to not only handle the interrupt but to schedule if
> scheduling would be useful.
>
> So I don't think we need all this RCU magic.  This really ought to be
> able to be simplified to:
>
> idtentry_exit();
>
> if (appropriate condition)
>   schedule();

This is exactly the kind of tinkering which causes all kinds of trouble.

idtentry_exit()

	if (user_mode(regs)) {
		prepare_exit_to_usermode(regs);
	} else if (regs->flags & X86_EFLAGS_IF) {
		/* Check kernel preemption, if enabled */
		if (IS_ENABLED(CONFIG_PREEMPTION)) {
                    ....
		}
		instrumentation_begin();
		/* Tell the tracer that IRET will enable interrupts */
		trace_hardirqs_on_prepare();
		lockdep_hardirqs_on_prepare(CALLER_ADDR0);
		instrumentation_end();
		rcu_irq_exit();
		lockdep_hardirqs_on(CALLER_ADDR0);
	} else {
		/* IRQ flags state is correct already. Just tell RCU */
		rcu_irq_exit();
	}

So in case IF is set then this already told the tracer and lockdep that
interrupts are enabled. And contrary to the ugly version this exit path
does not use rcu_irq_exit_preempt() which is there to warn about crappy
RCU state when trying to schedule.

So we went great length to sanitize _all_ of this and make it consistent
just to say: screw it for that xen thingy.

The extra checks and extra warnings for scheduling come with the
guarantee to bitrot when idtentry_exit() or any logic invoked from there
is changed. It's going to look like this:

	/*
         * If the below causes problems due to inconsistent state
         * or out of sync sanity checks, please complain to
         * luto@kernel.org directly.
         */
        idtentry_exit();

	if (user_mode(regs) || !(regs->flags & X86_FlAGS_IF))
        	return;

        if (!__this_cpu_read(xen_in_preemptible_hcall))
        	return;

        rcu_sanity_check_for_preemption();

        if (need_resched()) {
        	instrumentation_begin();
		xen_maybe_preempt_hcall();
		trace_hardirqs_on();
		instrumentation_end();
	}  	

Of course you need the extra rcu_sanity_check_for_preemption() function
just for this muck.

That's a true win on all ends? I don't think so.

Thanks,

        tglx