linux-kernel - Re: [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU

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Date:   Thu, 19 Jul 2018 14:54:56 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Waiman Long <longman@...hat.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v2] xen/spinlock: Don't use pvqspinlock if only 1 vCPU

On Thu, 19 Jul 2018, Waiman Long wrote:

>On a VM with only 1 vCPU, the locking fast paths will always be
>successful. In this case, there is no need to use the the PV qspinlock
>code which has higher overhead on the unlock side than the native
>qspinlock code.
>
>The xen_pvspin veriable is also turned off in this 1 vCPU case to
>eliminate unneeded pvqspinlock initialization in xen_init_lock_cpu()
>which is run after xen_init_spinlocks().

Wouldn't kvm also want this?

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index a37bda38d205..95aceb692010 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -457,7 +457,8 @@ static void __init sev_map_percpu_data(void)
 static void __init kvm_smp_prepare_cpus(unsigned int max_cpus)
 {
 	native_smp_prepare_cpus(max_cpus);
-	if (kvm_para_has_hint(KVM_HINTS_REALTIME))
+	if (num_possible_cpus() == 1 ||
+	    kvm_para_has_hint(KVM_HINTS_REALTIME))
 		static_branch_disable(&virt_spin_lock_key);
 }