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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham 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 74F09C432C2 for ; Wed, 25 Sep 2019 20:51:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4BDCE222C3 for ; Wed, 25 Sep 2019 20:51:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2393255AbfIYUvc (ORCPT ); Wed, 25 Sep 2019 16:51:32 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53728 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387977AbfIYUvb (ORCPT ); Wed, 25 Sep 2019 16:51:31 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 20C5D8AC6F5; Wed, 25 Sep 2019 20:51:31 +0000 (UTC) Received: from mail (ovpn-120-159.rdu2.redhat.com [10.10.120.159]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C303B10018F8; Wed, 25 Sep 2019 20:51:28 +0000 (UTC) Date: Wed, 25 Sep 2019 16:51:28 -0400 From: Andrea Arcangeli To: Christophe de Dinechin Cc: Vitaly Kuznetsov , Paolo Bonzini , "Dr. David Alan Gilbert" , Marcelo Tosatti , Peter Xu , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 15/17] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers Message-ID: <20190925205128.GB13637@redhat.com> References: <20190920212509.2578-1-aarcange@redhat.com> <20190920212509.2578-16-aarcange@redhat.com> <87o8zb8ik1.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.69]); Wed, 25 Sep 2019 20:51:31 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 25, 2019 at 01:03:32PM +0200, Christophe de Dinechin wrote: > > > > On 23 Sep 2019, at 11:31, Vitaly Kuznetsov wrote: > > > > Andrea Arcangeli > writes: > > > >> It's enough to check the exit value and issue a direct call to avoid > >> the retpoline for all the common vmexit reasons. > >> > >> Signed-off-by: Andrea Arcangeli > >> --- > >> arch/x86/kvm/vmx/vmx.c | 24 ++++++++++++++++++++++-- > >> 1 file changed, 22 insertions(+), 2 deletions(-) > >> > >> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > >> index a6e597025011..9aa73e216df2 100644 > >> --- a/arch/x86/kvm/vmx/vmx.c > >> +++ b/arch/x86/kvm/vmx/vmx.c > >> @@ -5866,9 +5866,29 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu) > >> } > >> > >> if (exit_reason < kvm_vmx_max_exit_handlers > >> - && kvm_vmx_exit_handlers[exit_reason]) > >> + && kvm_vmx_exit_handlers[exit_reason]) { > >> +#ifdef CONFIG_RETPOLINE > >> + if (exit_reason == EXIT_REASON_MSR_WRITE) > >> + return handle_wrmsr(vcpu); > >> + else if (exit_reason == EXIT_REASON_PREEMPTION_TIMER) > >> + return handle_preemption_timer(vcpu); > >> + else if (exit_reason == EXIT_REASON_PENDING_INTERRUPT) > >> + return handle_interrupt_window(vcpu); > >> + else if (exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT) > >> + return handle_external_interrupt(vcpu); > >> + else if (exit_reason == EXIT_REASON_HLT) > >> + return handle_halt(vcpu); > >> + else if (exit_reason == EXIT_REASON_PAUSE_INSTRUCTION) > >> + return handle_pause(vcpu); > >> + else if (exit_reason == EXIT_REASON_MSR_READ) > >> + return handle_rdmsr(vcpu); > >> + else if (exit_reason == EXIT_REASON_CPUID) > >> + return handle_cpuid(vcpu); > >> + else if (exit_reason == EXIT_REASON_EPT_MISCONFIG) > >> + return handle_ept_misconfig(vcpu); > >> +#endif > >> return kvm_vmx_exit_handlers[exit_reason](vcpu); > > > > I agree with the identified set of most common vmexits, however, this > > still looks a bit random. Would it be too much if we get rid of > > kvm_vmx_exit_handlers completely replacing this code with one switch()? > > Not sure, but if you do that, won’t the compiler generate a table and > bring you back to square one? Or is there a reason why the mitigation > is not needed for tables and indirect branches generated from switch > statements? When the kernel is built with retpolines the compiler is forbidden to use a table for any switch. I pointed out the relevant commit earlier in this thread. Instead the compiler will still try to bisect the exit_reason trying to make the cost more equal for all exit_reason and to reduce the number of checks, but we know the most likely exits so it should be better to prioritize the most frequent exit reasons. Thanks, Andrea