[PATCH 39/44] KVM: Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock

[Sean Christopherson]

From: Isaku Yamahata <isaku.yamahata at intel.com>

Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock now
that KVM hooks CPU hotplug during the ONLINE phase, which can sleep.
Previously, KVM hooked the STARTING phase, which is not allowed to sleep
and thus could not take kvm_lock (a mutex).

Explicitly disable preemptions/IRQs in the CPU hotplug paths as needed to
keep arch code happy, e.g. x86 expects IRQs to be disabled during hardware
enabling, and expects preemption to be disabled during hardware disabling.
There are no preemption/interrupt concerns in the hotplug path, i.e. the
extra disabling is done purely to allow x86 to keep its sanity checks,
which are targeted primiarily at the "enable/disable all" paths.

Opportunistically update KVM's locking documentation.

Signed-off-by: Isaku Yamahata <isaku.yamahata at intel.com>
Co-developed-by: Sean Christopherson <seanjc at google.com>
Signed-off-by: Sean Christopherson <seanjc at google.com>
---
 Documentation/virt/kvm/locking.rst | 18 ++++++------
 virt/kvm/kvm_main.c                | 44 +++++++++++++++++++++---------
 2 files changed, 40 insertions(+), 22 deletions(-)

diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
index 845a561629f1..4feaf527575b 100644
--- a/Documentation/virt/kvm/locking.rst
+++ b/Documentation/virt/kvm/locking.rst
@@ -9,6 +9,8 @@ KVM Lock Overview
 
 The acquisition orders for mutexes are as follows:
 
+- cpus_read_lock() is taken outside kvm_lock
+
 - kvm->lock is taken outside vcpu->mutex
 
 - kvm->lock is taken outside kvm->slots_lock and kvm->irq_lock
@@ -29,6 +31,8 @@ The acquisition orders for mutexes are as follows:
 
 On x86:
 
+- kvm_lock is taken outside kvm->mmu_lock
+
 - vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock
 
 - kvm->arch.mmu_lock is an rwlock.  kvm->arch.tdp_mmu_pages_lock and
@@ -216,15 +220,11 @@ time it will be set using the Dirty tracking mechanism described above.
 :Type:		mutex
 :Arch:		any
 :Protects:	- vm_list
-
-``kvm_count_lock``
-^^^^^^^^^^^^^^^^^^
-
-:Type:		raw_spinlock_t
-:Arch:		any
-:Protects:	- hardware virtualization enable/disable
-:Comment:	'raw' because hardware enabling/disabling must be atomic /wrt
-		migration.
+		- kvm_usage_count
+		- hardware virtualization enable/disable
+		- module probing (x86 only)
+:Comment:	KVM also disables CPU hotplug via cpus_read_lock() during
+		enable/disable.
 
 ``kvm->mn_invalidate_lock``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4e765ef9f4bd..c8d92e6c3922 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -100,7 +100,6 @@ EXPORT_SYMBOL_GPL(halt_poll_ns_shrink);
  */
 
 DEFINE_MUTEX(kvm_lock);
-static DEFINE_RAW_SPINLOCK(kvm_count_lock);
 LIST_HEAD(vm_list);
 
 static cpumask_var_t cpus_hardware_enabled;
@@ -5028,9 +5027,10 @@ static void hardware_enable_nolock(void *junk)
 
 static int kvm_online_cpu(unsigned int cpu)
 {
+	unsigned long flags;
 	int ret = 0;
 
-	raw_spin_lock(&kvm_count_lock);
+	mutex_lock(&kvm_lock);
 	/*
 	 * Abort the CPU online process if hardware virtualization cannot
 	 * be enabled. Otherwise running VMs would encounter unrecoverable
@@ -5039,13 +5039,16 @@ static int kvm_online_cpu(unsigned int cpu)
 	if (kvm_usage_count) {
 		WARN_ON_ONCE(atomic_read(&hardware_enable_failed));
 
+		local_irq_save(flags);
 		hardware_enable_nolock(NULL);
+		local_irq_restore(flags);
+
 		if (atomic_read(&hardware_enable_failed)) {
 			atomic_set(&hardware_enable_failed, 0);
 			ret = -EIO;
 		}
 	}
-	raw_spin_unlock(&kvm_count_lock);
+	mutex_unlock(&kvm_lock);
 	return ret;
 }
 
@@ -5061,10 +5064,13 @@ static void hardware_disable_nolock(void *junk)
 
 static int kvm_offline_cpu(unsigned int cpu)
 {
-	raw_spin_lock(&kvm_count_lock);
-	if (kvm_usage_count)
+	mutex_lock(&kvm_lock);
+	if (kvm_usage_count) {
+		preempt_disable();
 		hardware_disable_nolock(NULL);
-	raw_spin_unlock(&kvm_count_lock);
+		preempt_enable();
+	}
+	mutex_unlock(&kvm_lock);
 	return 0;
 }
 
@@ -5079,9 +5085,11 @@ static void hardware_disable_all_nolock(void)
 
 static void hardware_disable_all(void)
 {
-	raw_spin_lock(&kvm_count_lock);
+	cpus_read_lock();
+	mutex_lock(&kvm_lock);
 	hardware_disable_all_nolock();
-	raw_spin_unlock(&kvm_count_lock);
+	mutex_unlock(&kvm_lock);
+	cpus_read_unlock();
 }
 
 static int hardware_enable_all(void)
@@ -5097,7 +5105,7 @@ static int hardware_enable_all(void)
 	 * Disable CPU hotplug to prevent scenarios where KVM sees
 	 */
 	cpus_read_lock();
-	raw_spin_lock(&kvm_count_lock);
+	mutex_lock(&kvm_lock);
 
 	kvm_usage_count++;
 	if (kvm_usage_count == 1) {
@@ -5110,7 +5118,7 @@ static int hardware_enable_all(void)
 		}
 	}
 
-	raw_spin_unlock(&kvm_count_lock);
+	mutex_unlock(&kvm_lock);
 	cpus_read_unlock();
 
 	return r;
@@ -5716,6 +5724,15 @@ static void kvm_init_debug(void)
 
 static int kvm_suspend(void)
 {
+	/*
+	 * Secondary CPUs and CPU hotplug are disabled across the suspend/resume
+	 * callbacks, i.e. no need to acquire kvm_lock to ensure the usage count
+	 * is stable.  Assert that kvm_lock is not held as a paranoid sanity
+	 * check that the system isn't suspended when KVM is enabling hardware.
+	 */
+	lockdep_assert_not_held(&kvm_lock);
+	lockdep_assert_irqs_disabled();
+
 	if (kvm_usage_count)
 		hardware_disable_nolock(NULL);
 	return 0;
@@ -5723,10 +5740,11 @@ static int kvm_suspend(void)
 
 static void kvm_resume(void)
 {
-	if (kvm_usage_count) {
-		lockdep_assert_not_held(&kvm_count_lock);
+	lockdep_assert_not_held(&kvm_lock);
+	lockdep_assert_irqs_disabled();
+
+	if (kvm_usage_count)
 		hardware_enable_nolock(NULL);
-	}
 }
 
 static struct syscore_ops kvm_syscore_ops = {
-- 
2.38.1.431.g37b22c650d-goog