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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 D3062C433EF for ; Wed, 22 Sep 2021 21:33:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AF50C60F6D for ; Wed, 22 Sep 2021 21:33:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238064AbhIVVep (ORCPT ); Wed, 22 Sep 2021 17:34:45 -0400 Received: from mga14.intel.com ([192.55.52.115]:1378 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237770AbhIVVen (ORCPT ); Wed, 22 Sep 2021 17:34:43 -0400 X-IronPort-AV: E=McAfee;i="6200,9189,10115"; a="223352213" X-IronPort-AV: E=Sophos;i="5.85,315,1624345200"; d="scan'208";a="223352213" Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by fmsmga103.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Sep 2021 14:33:12 -0700 X-IronPort-AV: E=Sophos;i="5.85,315,1624345200"; d="scan'208";a="704191017" Received: from slaxmina-mobl1.amr.corp.intel.com (HELO [10.209.53.12]) ([10.209.53.12]) by fmsmga005-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Sep 2021 14:33:12 -0700 Subject: Re: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP To: Peter Zijlstra , Fenghua Yu Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Andy Lutomirski , Tony Luck , Lu Baolu , Joerg Roedel , Josh Poimboeuf , Dave Jiang , Jacob Jun Pan , Ashok Raj , Ravi V Shankar , iommu@lists.linux-foundation.org, x86 , linux-kernel References: <20210920192349.2602141-1-fenghua.yu@intel.com> <20210920192349.2602141-5-fenghua.yu@intel.com> <20210922210722.GV4323@worktop.programming.kicks-ass.net> <20210922211145.GF5106@worktop.programming.kicks-ass.net> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20210922211145.GF5106@worktop.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/22/21 2:11 PM, Peter Zijlstra wrote: >>> +static bool fixup_pasid_exception(void) >>> +{ >>> + if (!cpu_feature_enabled(X86_FEATURE_ENQCMD)) >>> + return false; >>> + >>> + return __fixup_pasid_exception(); >>> +} > That is, shouldn't the above at the very least decode the instruction > causing the #GP and check it's this ENQCMD thing? To reiterate: on systems with no X86_FEATURE_ENQCMD, there is basically no additional overhead. It isn't worth doing decoding there. On systems with X86_FEATURE_ENQCMD, but where it is unused, the #GP handler gets some new overhead on every #GP. Basically: > + pasid = current->mm->pasid; > + if (pasid == PASID_DISABLED) > + return false; That's still pretty cheap. Probably not worth doing decoding there either. So, that leaves us with if you are: 1. On system with X86_FEATURE_ENQCMD 2. In a process/mm that has an allocated pasid 3. Your *task* does not have the MSR set 4. You get a #GP for some other reason Then, you'll do this double-#GP dance. So, instruction decoding could absolutely be added between steps 3 and 4. It would absolutely save doing the double-#GP in cases where 1/2/3 are met. But, I wouldn't move it up above and of the 1/2/3 checks because they're way cheaper than instruction decoding. In the end, it didn't seem worth it to me to be optimizing a relatively rare path which 99% of the time ends up in a crash. If you want instruction decoding in here, though, just say the word. :)