Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751498AbbHSRSR (ORCPT ); Wed, 19 Aug 2015 13:18:17 -0400 Received: from mail-wi0-f174.google.com ([209.85.212.174]:38907 "EHLO mail-wi0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750797AbbHSRSP (ORCPT ); Wed, 19 Aug 2015 13:18:15 -0400 Date: Wed, 19 Aug 2015 19:18:12 +0200 From: Frederic Weisbecker To: Andy Lutomirski Cc: Denys Vlasenko , Rik van Riel , Borislav Petkov , Peter Zijlstra , Brian Gerst , Denys Vlasenko , Kees Cook , Thomas Gleixner , Oleg Nesterov , Andrew Lutomirski , Linus Torvalds , Ingo Molnar , "H. Peter Anvin" , "linux-kernel@vger.kernel.org" , "linux-tip-commits@vger.kernel.org" Subject: Re: [tip:x86/asm] x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code Message-ID: <20150819171811.GB21717@lerouge> References: <60e90901eee611e59e958bfdbbe39969b4f88fe5.1435952415.git.luto@kernel.org> <20150811223827.GB15639@lerouge> <20150811232234.GD15639@lerouge> <20150812133217.GB21542@lerouge> <20150818223409.GB12858@lerouge> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3097 Lines: 71 On Tue, Aug 18, 2015 at 03:40:20PM -0700, Andy Lutomirski wrote: > On Tue, Aug 18, 2015 at 3:34 PM, Frederic Weisbecker wrote: > >> If we switched back to exception_enter, we'd have to remember the > >> previous state, and, with a single exception right now, I think that's > >> unnecessary. > >> > >> I think there are only three states we can be in at exception entry: > >> user (and user_mode(regs)), kernel (and kernel_mode(regs)), or > >> NMI-like. > > > > But we can have user && (!user_mode(regs)) if exception happens on exception > > entry code. > > I sure hope not, unless it nests inside an NMI-like thing. It's > conceivable that this might happen due to perf NMIs causing a failed > MSR read or similar. We might need to relax the assertions to check > that we're either in kernel or NMI context. If so, that's > straightforward. Meanwhile no one has reported this happening. But we can still have #DB on entry code right? We blocked breakpoints on entry code (I still don't get why and it looks to me like an overkill) but we still have watchpoints. > > > > >> In the user case, the new code is correct. In the kernel > >> case, the new code is also correct. In the NMI case (if we're nested > >> in an NMI or similar entry)) then it is and was the responsibility of > >> the NMI-like entry to call rcu_nmi_enter(), and things that nest > >> inside that shouldn't touch context tracking (with the possible > >> exception of calling rcu_nmi_enter() again). > >> > >> In current -tip, there's a slight hole in this due to syscalls, and I'll fix it. > > > > There must be a check for context tracking enabled anyway. So why can't > > we just just do in exception entry code: > > > > if (exception_slow_path()) { > > exception_enter() > > exception_handler() > > exception_exit() > > } else { > > normal stuff > > } > > > > Especially if we can manage to implement static keys in ASM, this will sum up to > > a single one. > > There isn't really an exception slow path. There's already a branch > for user vs kernel (in the CPL sense), and with my patches, there's no > additional branch for previous context tracking state. But an exception slow path based on static key would the most lightweight thing for context tracking off-case (which is 99.9999% of usecases) and we would keep it robust (ie: no need to enumerate all the fragile non-possibility for an exception in entry code to get it safe). > > So now we can't set a breakpoint on syscall entry anymore? > > > > I'm still nervous with all that. > > We haven't done anything that would make breakpoints on syscall entry > less safe than they were, but we now disallow the breakpoints. In the > future, we might take advantage of that change. I still don't get the reason of that. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/