linux-kernel - Re: [PATCH 1/1] locking/qspinlock/x86: Avoid test-and-set when PV

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Message-ID: <20171031170254.GA12738@u40b0340c692b58f6553c.ant.amazon.com>
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