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[84.3.50.134]) by smtp.gmail.com with ESMTPSA id 189sm330432wmc.7.2019.11.21.09.12.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 21 Nov 2019 09:12:16 -0800 (PST) Date: Thu, 21 Nov 2019 18:12:14 +0100 From: Ingo Molnar To: Peter Zijlstra Cc: Fenghua Yu , Thomas Gleixner , Ingo Molnar , Borislav Petkov , H Peter Anvin , Tony Luck , Ashok Raj , Ravi V Shankar , linux-kernel , x86 Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter Message-ID: <20191121171214.GD12042@gmail.com> References: <1574297603-198156-1-git-send-email-fenghua.yu@intel.com> <1574297603-198156-7-git-send-email-fenghua.yu@intel.com> <20191121060444.GA55272@gmail.com> <20191121130153.GS4097@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191121130153.GS4097@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Peter Zijlstra wrote: > On Thu, Nov 21, 2019 at 07:04:44AM +0100, Ingo Molnar wrote: > > * Fenghua Yu wrote: > > > > + split_lock_detect > > > + [X86] Enable split lock detection > > > + This is a real time or debugging feature. When enabled > > > + (and if hardware support is present), atomic > > > + instructions that access data across cache line > > > + boundaries will result in an alignment check exception. > > > + When triggered in applications the kernel will send > > > + SIGBUS. The kernel will panic for a split lock in > > > + OS code. > > > > It would be really nice to be able to enable/disable this runtime as > > well, has this been raised before, and what was the conclusion? > > It has, previous versions had that. Somehow a lot of things went missing > and we're back to a broken neutered useless mess. > > The problem appears to be that due to hardware design the feature cannot > be virtualized, and instead of then disabling it when a VM runs/exists > they just threw in the towel and went back to useless mode.. :-( > > This feature MUST be default enabled, otherwise everything will > be/remain broken and we'll end up in the situation where you can't use > it even if you wanted to. Agreed. > And I can't be arsed to look it up, but we've been making this very > same argument since very early (possible the very first) version. Yeah, I now have a distinct deja vu... Thanks, Ingo