KVM: Disable CPU hotplug during hardware enabling/disabling

Disable CPU hotplug when enabling/disabling hardware to prevent the
corner case where if the following sequence occurs:

  1. A hotplugged CPU marks itself online in cpu_online_mask
  2. The hotplugged CPU enables interrupt before invoking KVM's ONLINE
     callback
  3  hardware_{en,dis}able_all() is invoked on another CPU

the hotplugged CPU will be included in on_each_cpu() and thus get sent
through hardware_{en,dis}able_nolock() before kvm_online_cpu() is called.

        start_secondary { ...
                set_cpu_online(smp_processor_id(), true); <- 1
                ...
                local_irq_enable();  <- 2
                ...
                cpu_startup_entry(CPUHP_AP_ONLINE_IDLE); <- 3
        }

KVM currently fudges around this race by keeping track of which CPUs have
done hardware enabling (see commit 1b6c016818a5 "KVM: Keep track of which
cpus have virtualization enabled"), but that's an inefficient, convoluted,
and hacky solution.

Signed-off-by: Chao Gao <chao.gao@intel.com>
[sean: split to separate patch, write changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221130230934.1014142-43-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ee1005cb99e1..2d6203ecab88 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -5167,15 +5167,26 @@ static void hardware_disable_all_nolock(void)
 
 static void hardware_disable_all(void)
 {
+	cpus_read_lock();
 	raw_spin_lock(&kvm_count_lock);
 	hardware_disable_all_nolock();
 	raw_spin_unlock(&kvm_count_lock);
+	cpus_read_unlock();
 }
 
 static int hardware_enable_all(void)
 {
 	int r = 0;
 
+	/*
+	 * When onlining a CPU, cpu_online_mask is set before kvm_online_cpu()
+	 * is called, and so on_each_cpu() between them includes the CPU that
+	 * is being onlined.  As a result, hardware_enable_nolock() may get
+	 * invoked before kvm_online_cpu(), which also enables hardware if the
+	 * usage count is non-zero.  Disable CPU hotplug to avoid attempting to
+	 * enable hardware multiple times.
+	 */
+	cpus_read_lock();
 	raw_spin_lock(&kvm_count_lock);
 
 	kvm_usage_count++;
@@ -5190,6 +5201,7 @@ static int hardware_enable_all(void)
 	}
 
 	raw_spin_unlock(&kvm_count_lock);
+	cpus_read_unlock();
 
 	return r;
 }