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

[Davidlohr Bueso <dave@xxxxxxxxxxxx>]

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);
}

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