Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965912AbeAJP5M (ORCPT + 1 other); Wed, 10 Jan 2018 10:57:12 -0500 Received: from mx1.redhat.com ([209.132.183.28]:27166 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932448AbeAJP5K (ORCPT ); Wed, 10 Jan 2018 10:57:10 -0500 Subject: Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest To: "Woodhouse, David" , "konrad.wilk@oracle.com" Cc: "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "jmattson@google.com" , "arjan@linux.intel.com" , "Liguori, Anthony" , "nadav.amit@gmail.com" , "x86@kernel.org" , "liran.alon@oracle.com" , "bp@alien8.de" , "thomas.lendacky@amd.com" , "rkrcmar@redhat.com" References: <222d0a6b-820f-4d7c-a616-ac89f77c3c09@default> <834e9b46-5d59-a81e-8cda-5f576964e1cb@redhat.com> <9360a280-228d-26d9-5561-6688aa67881c@linux.intel.com> <91d41f66-d744-e8b0-89f0-a167d3a3918c@redhat.com> <20180110154138.GE29272@char.us.oracle.com> <1515599333.22302.195.camel@amazon.co.uk> From: Paolo Bonzini Message-ID: <660da944-6e24-5ed6-6d85-94efd67180c9@redhat.com> Date: Wed, 10 Jan 2018 16:56:42 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0 MIME-Version: 1.0 In-Reply-To: <1515599333.22302.195.camel@amazon.co.uk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Wed, 10 Jan 2018 15:57:05 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: On 10/01/2018 16:48, Woodhouse, David wrote: >> >> And what is the point of this "always set IBRS=1" then? Are there >> some other things lurking in the shadows? > Yes. *FUTURE* CPUs will have a mode where you can just set IBRS and > leave it set for ever and not worry about any of this, and the > performance won't even suck. > > Quite why it's still an option you have to set in an MSR, and not just > a feature bit that they advertise and do it unconditionally, I have no > idea. But apparently that's the plan. And again---why you still need IBPBs. That also escapes me. I wouldn't be surprised if that's just a trick to sneak it in a generation earlier... Paolo