From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751666AbcGORyi (ORCPT ); Fri, 15 Jul 2016 13:54:38 -0400 Received: from mail-oi0-f51.google.com ([209.85.218.51]:35863 "EHLO mail-oi0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751241AbcGORye (ORCPT ); Fri, 15 Jul 2016 13:54:34 -0400 MIME-Version: 1.0 In-Reply-To: <7f189511-4432-8208-28f0-701f1d05e91e@redhat.com> References: <1468455397-22003-1-git-send-email-dmatlack@google.com> <7f189511-4432-8208-28f0-701f1d05e91e@redhat.com> From: David Matlack Date: Fri, 15 Jul 2016 10:54:13 -0700 Message-ID: Subject: Re: [PATCH] kvm: x86: nVMX: maintain internal copy of current VMCS To: Paolo Bonzini Cc: kvm list , Jim Mattson , Peter Feiner , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 14, 2016 at 1:33 AM, Paolo Bonzini wrote: > > > On 14/07/2016 02:16, David Matlack wrote: >> KVM maintains L1's current VMCS in guest memory, at the guest physical >> page identified by the argument to VMPTRLD. This makes hairy >> time-of-check to time-of-use bugs possible,as VCPUs can be writing >> the the VMCS page in memory while KVM is emulating VMLAUNCH and >> VMRESUME. >> >> The spec documents that writing to the VMCS page while it is loaded is >> "undefined". Therefore it is reasonable to load the entire VMCS into >> an internal cache during VMPTRLD and ignore writes to the VMCS page >> -- the guest should be using VMREAD and VMWRITE to access the current >> VMCS. >> >> To adhere to the spec, KVM should flush the current VMCS during VMPTRLD, >> and the target VMCS during VMCLEAR (as given by the operand to VMCLEAR). >> Since this implementation of VMCS caching only maintains the the current >> VMCS, VMCLEAR will only do a flush if the operand to VMCLEAR is the >> current VMCS pointer. >> >> KVM will also flush during VMXOFF, which is not mandated by the spec, >> but also not in conflict with the spec. >> >> Signed-off-by: David Matlack > > This is a good change. There is another change that is possible on top: > with this change you don't need current_vmcs12/current_vmcs12_page at > all, I think. You can just use current_vmptr and kvm_read/write_guest > to write back the VMCS12, possibly the cached variants. Good catch, I agree they can be removed. > > Of course this would just be a small simplification, so I'm applying the > patch as is to kvm/next. SGTM. Thanks for the review. > > Thanks, > > Paolo