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Re: [Xen-devel] [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)
-       if (kvm_para_has_hint(KVM_HINTS_REALTIME))
+       if (num_possible_cpus() == 1 ||
+           kvm_para_has_hint(KVM_HINTS_REALTIME))

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