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

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Message-ID: <3908561D78D1C84285E8C5FCA982C28F328116D8@ORSMSX114.amr.corp.intel.com>
Date:	Wed, 21 May 2014 22:01:36 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>
CC:	Jiri Kosina <jkosina@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>
Subject: RE: [RFC] x86_64: A real proposal for iret-less return to kernel

> 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