KVM: x86: Optimize kvm->lock and SRCU interaction (KVM_X86_SET_MSR_FILTER)

Reduce time spent holding kvm->lock: unlock mutex before calling
synchronize_srcu().  There is no need to hold kvm->lock until all vCPUs
have been kicked, KVM only needs to guarantee that all vCPUs will switch
to the new filter before exiting to userspace.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20230107001256.2365304-3-mhal@rbox.co
[sean: expand changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: mmhal

arch/x86/kvm/x86.c

@@ -6497,12 +6497,12 @@ static int kvm_vm_ioctl_set_msr_filter(struct kvm *kvm,
 	old_filter = srcu_dereference_check(kvm->arch.msr_filter, &kvm->srcu, 1);
 
 	rcu_assign_pointer(kvm->arch.msr_filter, new_filter);
+	mutex_unlock(&kvm->lock);
 	synchronize_srcu(&kvm->srcu);
 
 	kvm_free_msr_filter(old_filter);
 
 	kvm_make_all_cpus_request(kvm, KVM_REQ_MSR_FILTER_CHANGED);
-	mutex_unlock(&kvm->lock);
 
 	return 0;
 }