Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751093AbcDBU6l (ORCPT ); Sat, 2 Apr 2016 16:58:41 -0400 Received: from mail-oi0-f46.google.com ([209.85.218.46]:33565 "EHLO mail-oi0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750766AbcDBU6j (ORCPT ); Sat, 2 Apr 2016 16:58:39 -0400 MIME-Version: 1.0 In-Reply-To: <20160402204752.GC2538@pd.tnic> References: <4085070316fc3ab29538d3fcfe282648d1d4ee2e.1459605520.git.luto@kernel.org> <20160402183919.GA2538@pd.tnic> <20160402204752.GC2538@pd.tnic> From: Andy Lutomirski Date: Sat, 2 Apr 2016 13:58:19 -0700 Message-ID: Subject: Re: [PATCH v5 3/9] x86/head: Move early exception panic code into early_fixup_exception To: Borislav Petkov Cc: 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 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1467 Lines: 57 [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? --Andy