[PATCH v2 42/50] KVM: Disable CPU hotplug during hardware enabling/disabling

[Sean Christopherson <seanjc@xxxxxxxxxx>]

From: Chao Gao <chao.gao@xxxxxxxxx>

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@xxxxxxxxx>
[sean: split to separate patch, write changelog]
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c  | 11 ++++++++++-
 virt/kvm/kvm_main.c | 12 ++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index dad30097f0c3..d2ad383da998 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9281,7 +9281,16 @@ static inline void kvm_ops_update(struct kvm_x86_init_ops *ops)
 
 static int kvm_x86_check_processor_compatibility(void)
 {
-	struct cpuinfo_x86 *c = &cpu_data(smp_processor_id());
+	int cpu = smp_processor_id();
+	struct cpuinfo_x86 *c = &cpu_data(cpu);
+
+	/*
+	 * Compatibility checks are done when loading KVM and when enabling
+	 * hardware, e.g. during CPU hotplug, to ensure all online CPUs are
+	 * compatible, i.e. KVM should never perform a compatibility check on
+	 * an offline CPU.
+	 */
+	WARN_ON(!cpu_online(cpu));
 
 	if (__cr4_reserved_bits(cpu_has, c) !=
 	    __cr4_reserved_bits(cpu_has, &boot_cpu_data))
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index f26ea779710a..d985b24c423b 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -5098,15 +5098,26 @@ 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++;
@@ -5121,6 +5132,7 @@ static int hardware_enable_all(void)
 	}
 
 	raw_spin_unlock(&kvm_count_lock);
+	cpus_read_unlock();
 
 	return r;
 }
-- 
2.38.1.584.g0f3c55d4c2-goog