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Date: Tue, 31 Oct 2017 10:02:54 -0700 From: Eduardo Valentin <eduval@...zon.com> To: Radim Krčmář <rkrcmar@...hat.com> CC: Eduardo Valentin <eduval@...zon.com>, Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Waiman Long <longman@...hat.com>, <kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>, "Jan H . Schoenherr" <jschoenh@...zon.de>, Anthony Liguori <aliguori@...zon.com>, <msw@...zon.com> Subject: Re: [PATCH 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set 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
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