Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755314AbcDDLvd (ORCPT ); Mon, 4 Apr 2016 07:51:33 -0400 Received: from mx2.suse.de ([195.135.220.15]:39216 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752378AbcDDLvb (ORCPT ); Mon, 4 Apr 2016 07:51:31 -0400 Date: Mon, 4 Apr 2016 13:52:06 +0200 From: Jan Kara To: Andy Lutomirski Cc: Borislav Petkov , Andy Lutomirski , X86 ML , Paolo Bonzini , Peter Zijlstra , KVM list , Arjan van de Ven , xen-devel , "linux-kernel@vger.kernel.org" , Linus Torvalds , Andrew Morton , Jan Kara , pmladek@suse.cz Subject: Re: [PATCH v5 3/9] x86/head: Move early exception panic code into early_fixup_exception Message-ID: <20160404115206.GG8372@quack.suse.cz> References: <4085070316fc3ab29538d3fcfe282648d1d4ee2e.1459605520.git.luto@kernel.org> <20160402183919.GA2538@pd.tnic> <20160402204752.GC2538@pd.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2256 Lines: 72 On Sat 02-04-16 13:58:19, Andy Lutomirski wrote: > [cc Jan Kara] > > On Sat, Apr 2, 2016 at 1:47 PM, Borislav Petkov wrote: > > On Sat, Apr 02, 2016 at 01:13:37PM -0700, Andy Lutomirski wrote: > >> Given that I this isn't really a regression with my patches (it > >> probably never worked much better on 32-bit and the regs never would > >> have shown at all on 64-bit), > > > > You're right. That thing calls printk *and* early_printk, WTF: > > > > #ifdef CONFIG_EARLY_PRINTK > > > > call early_printk > > ... > > > > call dump_stack > > > > ... > > > > call __print_symbol > > > > those last two call printk. Great. > > > >> I propose a different approach: make > >> printk work earlier. Something like: > >> > >> if (early) { > >> early_printk(args); > >> } > >> > >> or early_vprintk or whatever. > >> > >> If the cost of a branch mattered, this could be alternative-patched > >> out later on, but that seems silly. I also bet that a more sensible > >> fallback could be created in which printk would try to use an early > >> console if there's no real console. > > > > So how about this: > > > > printk() does > > > > vprintk_func = this_cpu_read(printk_func); > > > > and that's > > > > DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default > > > > I guess we can make that function be early_printk-something and once > > printk is initialized, we overwrite it with vprintk_default. > > > > Elegant and no need for if branches and alternatives. > > > > Hmmm. > > Jan, IIRC you were looking at printk recently-ish. Any thoughts here? Sounds like a good idea to me. I've also consulted this with Petr Mladek (added to CC) who is using printk_func per-cpu variable in his printk-from-NMI patches and he also doesn't see a problem with this. I was just wondering about one thing - this way we add more early printks if I understand your intention right. Are we guaranteed that they happen only from a single CPU? Because currently there is no locking in early_printk() and thus we can end up writing to early console several messages in parallel from different CPUs. Not sure what's going to happen in that case... Honza -- Jan Kara SUSE Labs, CR