From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9BE32C43331 for ; Sat, 28 Mar 2020 16:32:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6AAF8206F6 for ; Sat, 28 Mar 2020 16:32:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727336AbgC1QcC (ORCPT ); Sat, 28 Mar 2020 12:32:02 -0400 Received: from mga07.intel.com ([134.134.136.100]:41730 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726205AbgC1QcB (ORCPT ); Sat, 28 Mar 2020 12:32:01 -0400 IronPort-SDR: R3J+PJG7vba9ujKPlC1WE5LfxgVHxoiSIyUunV7wm1ZuPRRG3PIHaVFh5UnvRx2duKuBYT2OaO U/pnpiKQEEQg== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga004.jf.intel.com ([10.7.209.38]) by orsmga105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 28 Mar 2020 09:32:01 -0700 IronPort-SDR: 3YgAP+y5LhQ1XSXQE7XhXK5YtO7kUL4CmFFbPQOqrtQr28IqWBb9cCKLsilwvR9TvVtYPeFnw8 R/X7tfLPu+NQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.72,317,1580803200"; d="scan'208";a="394686164" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.202]) by orsmga004.jf.intel.com with ESMTP; 28 Mar 2020 09:32:01 -0700 Date: Sat, 28 Mar 2020 09:32:01 -0700 From: Sean Christopherson To: Xiaoyao Li Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , hpa@zytor.com, x86@kernel.org, linux-kernel@vger.kernel.org, Paolo Bonzini , luto@kernel.org, Peter Zijlstra , Arvind Sankar , Fenghua Yu , Tony Luck Subject: Re: [PATCH v7 1/2] x86/split_lock: Rework the initialization flow of split lock detection Message-ID: <20200328163201.GI8104@linux.intel.com> References: <20200325030924.132881-1-xiaoyao.li@intel.com> <20200325030924.132881-2-xiaoyao.li@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200325030924.132881-2-xiaoyao.li@intel.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 25, 2020 at 11:09:23AM +0800, Xiaoyao Li wrote: > static void __init split_lock_setup(void) > { > + enum split_lock_detect_state state = sld_warn; > char arg[20]; > int i, ret; > > - setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT); > - sld_state = sld_warn; > + if (!split_lock_verify_msr(false)) { > + pr_info("MSR access failed: Disabled\n"); A few nits on the error handling. The error message for this is a bit wonky, lots of colons and it's not super clear what "Disabled" refers to. [ 0.000000] x86/split lock detection: MSR access failed: Disabled Maybe this, so that it reads "split lock detection disabled because the MSR access failed". pr_info("Disabled, MSR access failed\n"); And rather than duplicate the error message, maybe use a goto, e.g. if (!split_lock_verify_msr(false)) goto msr_failed; ... if (!split_lock_verify_msr(true)) goto msr_failed; > + return; > + } > > ret = cmdline_find_option(boot_command_line, "split_lock_detect", > arg, sizeof(arg)); > if (ret >= 0) { > for (i = 0; i < ARRAY_SIZE(sld_options); i++) { > if (match_option(arg, ret, sld_options[i].option)) { > - sld_state = sld_options[i].state; > + state = sld_options[i].state; > break; > } > } > } > > - switch (sld_state) { > + switch (state) { > case sld_off: > pr_info("disabled\n"); > - break; > - > + return; > case sld_warn: > pr_info("warning about user-space split_locks\n"); > break; > - > case sld_fatal: > pr_info("sending SIGBUS on user-space split_locks\n"); > break; > } > + > + if (!split_lock_verify_msr(true)) { > + pr_info("MSR access failed: Disabled\n"); > + return; > + } > + > + sld_state = state; > + setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT); > } > > /* > - * Locking is not required at the moment because only bit 29 of this > - * MSR is implemented and locking would not prevent that the operation > - * of one thread is immediately undone by the sibling thread. > - * Use the "safe" versions of rdmsr/wrmsr here because although code > - * checks CPUID and MSR bits to make sure the TEST_CTRL MSR should > - * exist, there may be glitches in virtualization that leave a guest > - * with an incorrect view of real h/w capabilities. > + * MSR_TEST_CTRL is per core, but we treat it like a per CPU MSR. Locking > + * is not implemented as one thread could undo the setting of the other > + * thread immediately after dropping the lock anyway. > */ > -static bool __sld_msr_set(bool on) > +static void sld_update_msr(bool on) > { > u64 test_ctrl_val; > > - if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val)) > - return false; > + rdmsrl(MSR_TEST_CTRL, test_ctrl_val); > > if (on) > test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT; > else > test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT; > > - return !wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val); > + wrmsrl(MSR_TEST_CTRL, test_ctrl_val); > } > > static void split_lock_init(void) > { > - if (sld_state == sld_off) > - return; > - > - if (__sld_msr_set(true)) > - return; > - > - /* > - * If this is anything other than the boot-cpu, you've done > - * funny things and you get to keep whatever pieces. > - */ > - pr_warn("MSR fail -- disabled\n"); > - sld_state = sld_off; > + split_lock_verify_msr(sld_state != sld_off); I think it'd be worth a WARN_ON() if this fails with sld_state != off. If the WRMSR fails, then presumably SLD is off when it's expected to be on. The implied WARN on the unsafe WRMSR in sld_update_msr() won't fire unless a task generates an #AC on a non-buggy core and then gets migrated to the buggy core. Even if the WARNs are redundant, if something is wrong it'd be a lot easier for a user to triage/debug if there is a WARN in boot as opposed to a runtime WARN that requires a misbehaving application and scheduler behavior. > } > > bool handle_user_split_lock(struct pt_regs *regs, long error_code) > @@ -1071,7 +1083,7 @@ bool handle_user_split_lock(struct pt_regs *regs, long error_code) > * progress and set TIF_SLD so the detection is re-enabled via > * switch_to_sld() when the task is scheduled out. > */ > - __sld_msr_set(false); > + sld_update_msr(false); > set_tsk_thread_flag(current, TIF_SLD); > return true; > } > @@ -1085,7 +1097,7 @@ bool handle_user_split_lock(struct pt_regs *regs, long error_code) > */ > void switch_to_sld(unsigned long tifn) > { > - __sld_msr_set(!(tifn & _TIF_SLD)); > + sld_update_msr(!(tifn & _TIF_SLD)); > } > > #define SPLIT_LOCK_CPU(model) {X86_VENDOR_INTEL, 6, model, X86_FEATURE_ANY} > -- > 2.20.1 >