RE: [RFC] x86_64: A real proposal for iret-less return to kernel

[Luck, Tony]

> But sending signals from #MC context is definitely a bad idea. I think
> we had addressed this with irq_work at some point but my memory is very
> hazy.

We added code for recoverable errors to get out of the MC context
before trying to lookup the page and send the signal.  Bottom of
do_machine_check():

        if (cfg->tolerant < 3) {
                if (no_way_out)
                        mce_panic("Fatal machine check on current CPU", &m, msg);
                if (worst == MCE_AR_SEVERITY) {
                        /* schedule action before return to userland */
                        mce_save_info(m.addr, m.mcgstatus & MCG_STATUS_RIPV);
                        set_thread_flag(TIF_MCE_NOTIFY);
                } else if (kill_it) {
                        force_sig(SIGBUS, current);
                }
        }

That TIF_MCE_NOTIFY prevents the return to user mode, and we end up in mce_notify_process().

The "force_sig()" there is legacy code - and perhaps should just move off to mce_notify_process()
too (need to save "worst" so it will know what to do).

-Tony