From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932543AbeCFKQ6 (ORCPT ); Tue, 6 Mar 2018 05:16:58 -0500 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:51036 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750947AbeCFKQ5 (ORCPT ); Tue, 6 Mar 2018 05:16:57 -0500 Subject: Re: [PATCH RFC 0/3] x86/kvm: avoid expensive rdmsrs for FS/GS base MSRs To: Vitaly Kuznetsov , kvm@vger.kernel.org Cc: linux-kernel@vger.kernel.org, x86@kernel.org, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andy Lutomirski References: <20180302105503.24428-1-vkuznets@redhat.com> From: Paolo Bonzini Message-ID: <4ed6b5fb-91fd-67c5-5adb-6206a24d643e@redhat.com> Date: Tue, 6 Mar 2018 11:16:49 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: <20180302105503.24428-1-vkuznets@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/03/2018 11:55, Vitaly Kuznetsov wrote: > Some time ago Paolo suggested to take a look at probably unneeded expensive > rdmsrs for FS/GS base MSR in vmx_save_host_state(). This is called on every > vcpu run when we need to handle vmexit in userspace. > > I have to admit I got a bit lost in our kernel FS/GS magic. I managed to > convince myself that in the well defined context (ioctl from userspace) > we can always get the required values from in-kernel variables and avoid > rdmsrs. But I may have missed something really important, thus RFC. > > My debug shows we're shaving off 240 cpu cycles (E5-2603 v3). > > In case these patches turn out to be worthwile AMD SVM can probably be > optimized the ame way. SVM is a bit different, because it uses VMLOAD/VMSAVE and so it doesn't have an equivalent of vmx_save_host_state(). Unfortunately, you cannot really eliminate VMLOAD/VMSAVE because it's the only way to load the hidden state of TR and LDTR---so you might as well use it to load FS and GS, even in 64-bit mode. In order to decrease the cost of vmload/vmsave, we could single out the simplest vmexit handlers and process them without even getting out of svm_vcpu_run, thus skipping all four of stgi/vmload/vmsave/clgi. However, this probably couldn't be done for the really common vmexits such as nested page fault, PIO or most MSR accesses. We _could_ do it for nested virt-related vmexits, but the advantage of that is getting smaller too, since Zen provides hardware support for nested GIF and nested VMLOAD/VMSAVE. Paolo > Vitaly Kuznetsov (3): > x86/kvm/vmx: read MSR_FS_BASE from current->thread > x86/kvm/vmx: read MSR_KERNEL_GS_BASE from current->thread > x86/kvm/vmx: avoid expensive rdmsr for MSR_GS_BASE > > arch/x86/kernel/cpu/common.c | 1 + > arch/x86/kvm/vmx.c | 7 ++++--- > 2 files changed, 5 insertions(+), 3 deletions(-) >