On 2019/10/4 22:52, Boris Ostrovsky wrote:
On 10/3/19 10:02 AM, Zhenzhong Duan wrote:
void __init kvm_spinlock_init(void)
{
- /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
- if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
- return;
-
- if (kvm_para_has_hint(KVM_HINTS_REALTIME))
+ /*
+ * Don't use the pvqspinlock code if no KVM_FEATURE_PV_UNHALT feature
+ * support, or there is REALTIME hints or only 1 vCPU.
+ */
+ if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT) ||
+ kvm_para_has_hint(KVM_HINTS_REALTIME) ||
+ num_possible_cpus() == 1) {
+ pr_info("PV spinlocks disabled\n");
return;
+ }
- /* Don't use the pvqspinlock code if there is only 1 vCPU. */
- if (num_possible_cpus() == 1)
+ if (nopvspin) {
+ pr_info("PV spinlocks disabled forced by \"nopvspin\" parameter.\n");
+ static_branch_disable(&virt_spin_lock_key);
Would it make sense to bring here the other site where the key is
disabled (in kvm_smp_prepare_cpus())?
Thanks for point out, I'll do it. Just not clear if I should do that in a separate patch,
there is a history about that code:
Its original place was here and then moved to kvm_smp_prepare_cpus() by below commit:
34226b6b ("KVM: X86: Fix setup the virt_spin_lock_key before static key get initialized")
which fixed jump_label_init() calling late issue.
Then 8990cac6 ("x86/jump_label: Initialize static branching early") move jump_label_init()
early, so commit 34226b6b could be reverted.
(and, in fact, shouldn't all of the checks that result in early return
above disable the key?)
I think we should enable he key for !kvm_para_has_feature(KVM_FEATURE_PV_UNHALT) case,
there is lock holder preemption issue as qspinlock is fair lock, virt_spin_lock()
is an optimization to that, imaging one pcpu running 10 vcpus of same guest
contending a same lock.
For kvm_para_has_hint(KVM_HINTS_REALTIME) case, hypervisor hints there is
no preemption and we should disable virt_spin_lock_key to use native qspinlock.
For the UP case, we don't care virt_spin_lock_key value.
For nopvspin case, we intentionally check native qspinlock code performance,
compare it with PV qspinlock, etc. So virt_spin_lock() optimization should be disabled.
Let me know if anything wrong with above understanding. Thanks
Zhenzhong
