From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751781AbeCNQMX (ORCPT ); Wed, 14 Mar 2018 12:12:23 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:33467 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751660AbeCNQMS (ORCPT ); Wed, 14 Mar 2018 12:12:18 -0400 X-Google-Smtp-Source: AG47ELtQYzPu13/713JmsZVfJGIBXpttg3UIpOzOSqcrcm0v1fcZM8rzIMwbl91nAdGjksaBxYjkhA== Subject: Re: [PATCH v2 0/2] x86/kvm: avoid expensive rdmsrs for FS/GS base MSRs To: Vitaly Kuznetsov , kvm@vger.kernel.org, x86@kernel.org Cc: linux-kernel@vger.kernel.org, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andy Lutomirski References: <20180313174805.11825-1-vkuznets@redhat.com> From: Paolo Bonzini Message-ID: <426fa4fd-7c4d-b6dc-cc19-b0034b7226aa@redhat.com> Date: Wed, 14 Mar 2018 17:12:13 +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: <20180313174805.11825-1-vkuznets@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 13/03/2018 18:48, Vitaly Kuznetsov wrote: > Changes since v1: > - Merged patches 1 and 2, skip savesegment() for FS and FS for x86_64 > as save_fsgs() already does that [Paolo Bonzini] > - Rename save_current_fsgs() -> save_fsgs_for_kvm() [Andy Lutomirski] > - Trimmed comments [Andy Lutomirski] > - Add Andy's A-b to what is now PATCH2 > > 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. > > vmx_save_host_state() is always called in a very well defined context > (ioctl from userspace) so we may try to get the required values for FS/GS > bases from in-kernel variables and avoid expensive rdmsrs. > > My debug shows we're shaving off 240 cpu cycles (E5-2603 v3). > > Vitaly Kuznetsov (2): > x86/kvm/vmx: read MSR_{FS,KERNEL_GS}_BASE from current->thread > x86/kvm/vmx: avoid expensive rdmsr for MSR_GS_BASE > > arch/x86/include/asm/processor.h | 10 ++++++++++ > arch/x86/kernel/cpu/common.c | 3 ++- > arch/x86/kernel/process_64.c | 14 ++++++++++++++ > arch/x86/kvm/vmx.c | 16 ++++++++++++---- > 4 files changed, 38 insertions(+), 5 deletions(-) > Queued, thanks. Paolo