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=-5.2 required=3.0 tests=BAYES_00, 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 544E4C43457 for ; Sat, 17 Oct 2020 05:14:18 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id B79622074A for ; Sat, 17 Oct 2020 05:14:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B79622074A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id DCEFD6B005D; Sat, 17 Oct 2020 01:14:16 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D803E900002; Sat, 17 Oct 2020 01:14:16 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C96AB6B0068; Sat, 17 Oct 2020 01:14:16 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0214.hostedemail.com [216.40.44.214]) by kanga.kvack.org (Postfix) with ESMTP id 9289E6B005D for ; Sat, 17 Oct 2020 01:14:16 -0400 (EDT) Received: from smtpin23.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 1F1CB1EE6 for ; Sat, 17 Oct 2020 05:14:16 +0000 (UTC) X-FDA: 77380251312.23.humor25_140a63727223 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin23.hostedemail.com (Postfix) with ESMTP id E7FDD37604 for ; Sat, 17 Oct 2020 05:14:15 +0000 (UTC) X-HE-Tag: humor25_140a63727223 X-Filterd-Recvd-Size: 4660 Received: from mga06.intel.com (mga06.intel.com [134.134.136.31]) by imf38.hostedemail.com (Postfix) with ESMTP for ; Sat, 17 Oct 2020 05:14:14 +0000 (UTC) IronPort-SDR: bM20T5wH6enla5OIlIoQmeCFlOexKhTT3QhDKtWYALWAMiPFA8XMp2H6rXinulwyoFNYnWK7P6 /TdTE842DokA== X-IronPort-AV: E=McAfee;i="6000,8403,9776"; a="228398366" X-IronPort-AV: E=Sophos;i="5.77,385,1596524400"; d="scan'208";a="228398366" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by orsmga104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 16 Oct 2020 22:14:11 -0700 IronPort-SDR: 3XRfBEMKtTlYf8Z8qW4rktOEmrz3tfwYHJY64FpmSJj9Yo7F5sdqW0j6CxcMd1S6SrD+tuozbQ 2OvTFTNMfadw== X-IronPort-AV: E=Sophos;i="5.77,385,1596524400"; d="scan'208";a="522523589" Received: from iweiny-desk2.sc.intel.com (HELO localhost) ([10.3.52.147]) by fmsmga005-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 16 Oct 2020 22:14:11 -0700 Date: Fri, 16 Oct 2020 22:14:10 -0700 From: Ira Weiny To: Peter Zijlstra Cc: Dave Hansen , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Andy Lutomirski , Fenghua Yu , x86@kernel.org, Dave Hansen , Dan Williams , Andrew Morton , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH RFC V3 4/9] x86/pks: Preserve the PKRS MSR on context switch Message-ID: <20201017051410.GW2046448@iweiny-DESK2.sc.intel.com> References: <20201009194258.3207172-1-ira.weiny@intel.com> <20201009194258.3207172-5-ira.weiny@intel.com> <429789d3-ab5b-49c3-65c3-f0fc30a12516@intel.com> <20201016111226.GN2611@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201016111226.GN2611@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.11.1 (2018-12-01) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Oct 16, 2020 at 01:12:26PM +0200, Peter Zijlstra wrote: > On Tue, Oct 13, 2020 at 11:31:45AM -0700, Dave Hansen wrote: > > > +/** > > > + * It should also be noted that the underlying WRMSR(MSR_IA32_PKRS) is not > > > + * serializing but still maintains ordering properties similar to WRPKRU. > > > + * The current SDM section on PKRS needs updating but should be the same as > > > + * that of WRPKRU. So to quote from the WRPKRU text: > > > + * > > > + * WRPKRU will never execute transiently. Memory accesses > > > + * affected by PKRU register will not execute (even transiently) > > > + * until all prior executions of WRPKRU have completed execution > > > + * and updated the PKRU register. > > > + */ > > > +void write_pkrs(u32 new_pkrs) > > > +{ > > > + u32 *pkrs; > > > + > > > + if (!static_cpu_has(X86_FEATURE_PKS)) > > > + return; > > > + > > > + pkrs = get_cpu_ptr(&pkrs_cache); > > > + if (*pkrs != new_pkrs) { > > > + *pkrs = new_pkrs; > > > + wrmsrl(MSR_IA32_PKRS, new_pkrs); > > > + } > > > + put_cpu_ptr(pkrs); > > > +} > > > > > > > It bugs me a *bit* that this is being called in a preempt-disabled > > region, but we still bother with the get/put_cpu jazz. Are there other > > future call-sites for this that aren't in preempt-disabled regions? > > So the previous version had a useful comment that got lost. Ok Looking back I see what happened... This comment... /* * PKRS is only temporarily changed during specific code paths. * Only a preemption during these windows away from the default * value would require updating the MSR. */ ... was added to pks_sched_in() but that got simplified down because cleaning up write_pkrs() made the code there obsolete. > This stuff > needs to fundamentally be preempt disabled, I agree, the update to the percpu cache value and MSR can not be torn. > so it either needs to > explicitly do so, or have an assertion that preemption is indeed > disabled. However, I don't think I understand clearly. Doesn't [get|put]_cpu_ptr() handle the preempt_disable() for us? Is it not sufficient to rely on that? Dave's comment seems to be the opposite where we need to eliminate preempt disable before calling write_pkrs(). FWIW I think I'm mistaken in my response to Dave regarding the preempt_disable() in pks_update_protection(). Ira