From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753903AbdJaRHR (ORCPT ); Tue, 31 Oct 2017 13:07:17 -0400 Received: from smtp-fw-9101.amazon.com ([207.171.184.25]:5612 "EHLO smtp-fw-9101.amazon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752177AbdJaRHP (ORCPT ); Tue, 31 Oct 2017 13:07:15 -0400 X-IronPort-AV: E=Sophos;i="5.44,324,1505779200"; d="scan'208";a="715137071" Date: Tue, 31 Oct 2017 10:02:54 -0700 From: Eduardo Valentin To: Radim =?utf-8?B?S3LEjW3DocWZ?= CC: Eduardo Valentin , Paolo Bonzini , Jonathan Corbet , Thomas Gleixner , "Ingo Molnar" , "H. Peter Anvin" , , Peter Zijlstra , Waiman Long , , , , "Jan H . Schoenherr" , Anthony Liguori , Subject: Re: [PATCH 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set Message-ID: <20171031170254.GA12738@u40b0340c692b58f6553c.ant.amazon.com> References: <1508805867-14583-1-git-send-email-eduval@amazon.com> <20171024111859.GA13316@flask> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20171024111859.GA13316@flask> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello Radim, On Tue, Oct 24, 2017 at 01:18:59PM +0200, Radim Krčmář wrote: > 2017-10-23 17:44-0700, Eduardo Valentin: > > Currently, the existing qspinlock implementation will fallback to > > test-and-set if the hypervisor has not set the PV_UNHALT flag. > > Where have you detected the main source of overhead with pinned VCPUs? > Makes me wonder if we couldn't improve general PV_UNHALT, This is essentially for cases of non-overcommitted vCPUs in which we want the instance vCPUs to run uninterrupted as much as possible. Here by disabling the PV_UNHALT, we avoid the accounting needed to properly do the PV_UNHALT hypercall, as the lock holder won't be preempted anyway for the 1:1 pin case. > > thanks. > > > This patch gives the opportunity to guest kernels to select > > between test-and-set and the regular queueu fair lock implementation > > based on the PV_DEDICATED KVM feature flag. When the PV_DEDICATED > > flag is not set, the code will still fall back to test-and-set, > > but when the PV_DEDICATED flag is set, the code will use > > the regular queue spinlock implementation. > > Some flag makes sense and we do want to make sure that userspaces don't > enable it in pass-through-cpuid mode. Did you mean something like: diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index 0099e10..8ceb503 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -211,7 +211,8 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu, } for (i = 0; i < cpuid->nent; i++) { vcpu->arch.cpuid_entries[i].function = cpuid_entries[i].function; - vcpu->arch.cpuid_entries[i].eax = cpuid_entries[i].eax; + vcpu->arch.cpuid_entries[i].eax = cpuid_entries[i].eax & + ~KVM_FEATURE_PV_DEDICATED; vcpu->arch.cpuid_entries[i].ebx = cpuid_entries[i].ebx; vcpu->arch.cpuid_entries[i].ecx = cpuid_entries[i].ecx; vcpu->arch.cpuid_entries[i].edx = cpuid_entries[i].edx; But I do not see any other KVM_FEATURE_* being enforced (e.g. PV_UNHALT). Do you mind elaborating a bit here? > -- All the best, Eduardo Valentin