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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 8C9B2C433E0 for ; Mon, 1 Feb 2021 00:34:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5889A64DDE for ; Mon, 1 Feb 2021 00:34:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229535AbhBAAeQ (ORCPT ); Sun, 31 Jan 2021 19:34:16 -0500 Received: from mga03.intel.com ([134.134.136.65]:16598 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229765AbhBAAdp (ORCPT ); Sun, 31 Jan 2021 19:33:45 -0500 IronPort-SDR: nLNcW8/7CjMtdMhpKAQL++NQhkxs2f1BmcmAeHkBcBuYK+et76kB8BqUCST3vA7deKZpHV/R0U GEvSsNPgcACA== X-IronPort-AV: E=McAfee;i="6000,8403,9881"; a="180693128" X-IronPort-AV: E=Sophos;i="5.79,391,1602572400"; d="scan'208";a="180693128" Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 31 Jan 2021 16:33:05 -0800 IronPort-SDR: DilUVe739Ls5IMR4qgDV2+BZZdW+nSoo5Fwh+x4f1HUlrxJwGvQ2YGQgJ5DE0kBga3S+Do7Bt0 6QbC11WC0dQw== X-IronPort-AV: E=Sophos;i="5.79,391,1602572400"; d="scan'208";a="368699782" Received: from kpeng-mobl1.amr.corp.intel.com (HELO khuang2-desk.gar.corp.intel.com) ([10.252.130.129]) by fmsmga008-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 31 Jan 2021 16:33:01 -0800 Date: Mon, 1 Feb 2021 13:32:59 +1300 From: Kai Huang To: Jarkko Sakkinen Cc: linux-sgx@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org, seanjc@google.com, luto@kernel.org, dave.hansen@intel.com, haitao.huang@intel.com, pbonzini@redhat.com, bp@alien8.de, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, jmattson@google.com, joro@8bytes.org, vkuznets@redhat.com, wanpengli@tencent.com Subject: Re: [RFC PATCH v3 16/27] KVM: VMX: Convert vcpu_vmx.exit_reason to a union Message-Id: <20210201133259.e0398c9f0b229fd79a8c16c6@intel.com> In-Reply-To: References: X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Sat, 30 Jan 2021 17:00:46 +0200 Jarkko Sakkinen wrote: > On Tue, Jan 26, 2021 at 10:31:37PM +1300, Kai Huang wrote: > > From: Sean Christopherson > > > > Convert vcpu_vmx.exit_reason from a u32 to a union (of size u32). The > > full VM_EXIT_REASON field is comprised of a 16-bit basic exit reason in > > bits 15:0, and single-bit modifiers in bits 31:16. > > > > Historically, KVM has only had to worry about handling the "failed > > VM-Entry" modifier, which could only be set in very specific flows and > > required dedicated handling. I.e. manually stripping the FAILED_VMENTRY > > bit was a somewhat viable approach. But even with only a single bit to > > worry about, KVM has had several bugs related to comparing a basic exit > > reason against the full exit reason store in vcpu_vmx. > > > > Upcoming Intel features, e.g. SGX, will add new modifier bits that can > > be set on more or less any VM-Exit, as opposed to the significantly more > > restricted FAILED_VMENTRY, i.e. correctly handling everything in one-off > > flows isn't scalable. Tracking exit reason in a union forces code to > > explicitly choose between consuming the full exit reason and the basic > > exit, and is a convenient way to document and access the modifiers. > > I *believe* that the change is correct but I dropped in the last paragraph > - most likely only because of lack of expertise in this area. > > I ask the most basic question: why SGX will add new modifier bits? Not 100% sure about your question. Assuming you are asking SGX hardware behavior, SGX architecture adds a new modifier bit (27) to Exit Reason, similar to new #PF.SGX bit. Please refer to SDM Volume 3, Chapter 27.2.1 Basic VM-Exit Information. Sean's commit msg already provides significant motivation of the change in this patch.