@@ -9,7 +9,7 @@ KVM Lock Overview
The acquisition orders for mutexes are as follows:
-- cpus_read_lock() is taken outside kvm_lock
+- cpus_read_lock() is taken outside kvm_lock and kvm_usage_lock
- kvm->lock is taken outside vcpu->mutex
@@ -24,6 +24,12 @@ The acquisition orders for mutexes are as follows:
are taken on the waiting side when modifying memslots, so MMU notifiers
must not take either kvm->slots_lock or kvm->slots_arch_lock.
+cpus_read_lock() vs kvm_lock:
+- Taking cpus_read_lock() outside of kvm_lock is problematic, despite that
+ being the official ordering, as it is quite easy to unknowingly trigger
+ cpus_read_lock() while holding kvm_lock. Use caution when walking vm_list,
+ e.g. avoid complex operations when possible.
+
For SRCU:
- ``synchronize_srcu(&kvm->srcu)`` is called inside critical sections
@@ -227,10 +233,17 @@ time it will be set using the Dirty tracking mechanism described above.
:Type: mutex
:Arch: any
:Protects: - vm_list
- - kvm_usage_count
+
+``kvm_usage_lock``
+^^^^^^^^^^^^^^^^^^
+
+:Type: mutex
+:Arch: any
+:Protects: - kvm_usage_count
- hardware virtualization enable/disable
-:Comment: KVM also disables CPU hotplug via cpus_read_lock() during
- enable/disable.
+:Comment: Exists because using kvm_lock leads to deadlock (see earlier comment
+ on cpus_read_lock() vs kvm_lock). Note, KVM also disables CPU hotplug via
+ cpus_read_lock() when enabling/disabling virtualization.
``kvm->mn_invalidate_lock``
^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -290,11 +303,12 @@ time it will be set using the Dirty tracking mechanism described above.
wakeup.
``vendor_module_lock``
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^^^
:Type: mutex
:Arch: x86
:Protects: loading a vendor module (kvm_amd or kvm_intel)
-:Comment: Exists because using kvm_lock leads to deadlock. cpu_hotplug_lock is
- taken outside of kvm_lock, e.g. in KVM's CPU online/offline callbacks, and
- many operations need to take cpu_hotplug_lock when loading a vendor module,
- e.g. updating static calls.
+:Comment: Exists because using kvm_lock leads to deadlock. kvm_lock is taken
+ in notifiers, e.g. __kvmclock_cpufreq_notifier(), that may be invoked while
+ cpu_hotplug_lock is held, e.g. from cpufreq_boost_trigger_state(), and many
+ operations need to take cpu_hotplug_lock when loading a vendor module, e.g.
+ updating static calls.
@@ -5576,6 +5576,7 @@ __visible bool kvm_rebooting;
EXPORT_SYMBOL_GPL(kvm_rebooting);
static DEFINE_PER_CPU(bool, hardware_enabled);
+static DEFINE_MUTEX(kvm_usage_lock);
static int kvm_usage_count;
static int __hardware_enable_nolock(void)
@@ -5608,10 +5609,10 @@ static int kvm_online_cpu(unsigned int cpu)
* be enabled. Otherwise running VMs would encounter unrecoverable
* errors when scheduled to this CPU.
*/
- mutex_lock(&kvm_lock);
+ mutex_lock(&kvm_usage_lock);
if (kvm_usage_count)
ret = __hardware_enable_nolock();
- mutex_unlock(&kvm_lock);
+ mutex_unlock(&kvm_usage_lock);
return ret;
}
@@ -5631,10 +5632,10 @@ static void hardware_disable_nolock(void *junk)
static int kvm_offline_cpu(unsigned int cpu)
{
- mutex_lock(&kvm_lock);
+ mutex_lock(&kvm_usage_lock);
if (kvm_usage_count)
hardware_disable_nolock(NULL);
- mutex_unlock(&kvm_lock);
+ mutex_unlock(&kvm_usage_lock);
return 0;
}
@@ -5650,9 +5651,9 @@ static void hardware_disable_all_nolock(void)
static void hardware_disable_all(void)
{
cpus_read_lock();
- mutex_lock(&kvm_lock);
+ mutex_lock(&kvm_usage_lock);
hardware_disable_all_nolock();
- mutex_unlock(&kvm_lock);
+ mutex_unlock(&kvm_usage_lock);
cpus_read_unlock();
}
@@ -5683,7 +5684,7 @@ static int hardware_enable_all(void)
* enable hardware multiple times.
*/
cpus_read_lock();
- mutex_lock(&kvm_lock);
+ mutex_lock(&kvm_usage_lock);
r = 0;
@@ -5697,7 +5698,7 @@ static int hardware_enable_all(void)
}
}
- mutex_unlock(&kvm_lock);
+ mutex_unlock(&kvm_usage_lock);
cpus_read_unlock();
return r;
@@ -5725,13 +5726,13 @@ 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 to ensure the system
- * isn't suspended while KVM is enabling hardware. Hardware enabling
- * can be preempted, but the task cannot be frozen until it has dropped
- * all locks (userspace tasks are frozen via a fake signal).
+ * callbacks, i.e. no need to acquire kvm_usage_lock to ensure the usage
+ * count is stable. Assert that kvm_usage_lock is not held to ensure
+ * the system isn't suspended while KVM is enabling hardware. Hardware
+ * enabling can be preempted, but the task cannot be frozen until it has
+ * dropped all locks (userspace tasks are frozen via a fake signal).
*/
- lockdep_assert_not_held(&kvm_lock);
+ lockdep_assert_not_held(&kvm_usage_lock);
lockdep_assert_irqs_disabled();
if (kvm_usage_count)
@@ -5741,7 +5742,7 @@ static int kvm_suspend(void)
static void kvm_resume(void)
{
- lockdep_assert_not_held(&kvm_lock);
+ lockdep_assert_not_held(&kvm_usage_lock);
lockdep_assert_irqs_disabled();
if (kvm_usage_count)
Use a dedicated mutex to guard kvm_usage_count to fix a potential deadlock on x86 due to a chain of locks and SRCU synchronizations. Translating the below lockdep splat, CPU1 #6 will wait on CPU0 #1, CPU0 #8 will wait on CPU2 #3, and CPU2 #7 will wait on CPU1 #4 (if there's a writer, due to the fairness of r/w semaphores). CPU0 CPU1 CPU2 1 lock(&kvm->slots_lock); 2 lock(&vcpu->mutex); 3 lock(&kvm->srcu); 4 lock(cpu_hotplug_lock); 5 lock(kvm_lock); 6 lock(&kvm->slots_lock); 7 lock(cpu_hotplug_lock); 8 sync(&kvm->srcu); Note, there are likely more potential deadlocks in KVM x86, e.g. the same pattern of taking cpu_hotplug_lock outside of kvm_lock likely exists with __kvmclock_cpufreq_notifier(): cpuhp_cpufreq_online() | -> cpufreq_online() | -> cpufreq_gov_performance_limits() | -> __cpufreq_driver_target() | -> __target_index() | -> cpufreq_freq_transition_begin() | -> cpufreq_notify_transition() | -> ... __kvmclock_cpufreq_notifier() But, actually triggering such deadlocks is beyond rare due to the combination of dependencies and timings involved. E.g. the cpufreq notifier is only used on older CPUs without a constant TSC, mucking with the NX hugepage mitigation while VMs are running is very uncommon, and doing so while also onlining/offlining a CPU (necessary to generate contention on cpu_hotplug_lock) would be even more unusual. The most robust solution to the general cpu_hotplug_lock issue is likely to switch vm_list to be an RCU-protected list, e.g. so that x86's cpufreq notifier doesn't to take kvm_lock. For now, settle for fixing the most blatant deadlock, as switching to an RCU-protected list is a much more involved change, but add a comment in locking.rst to call out that care needs to be taken when walking holding kvm_lock and walking vm_list. ====================================================== WARNING: possible circular locking dependency detected 6.10.0-smp--c257535a0c9d-pip #330 Tainted: G S O ------------------------------------------------------ tee/35048 is trying to acquire lock: ff6a80eced71e0a8 (&kvm->slots_lock){+.+.}-{3:3}, at: set_nx_huge_pages+0x179/0x1e0 [kvm] but task is already holding lock: ffffffffc07abb08 (kvm_lock){+.+.}-{3:3}, at: set_nx_huge_pages+0x14a/0x1e0 [kvm] which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #3 (kvm_lock){+.+.}-{3:3}: __mutex_lock+0x6a/0xb40 mutex_lock_nested+0x1f/0x30 kvm_dev_ioctl+0x4fb/0xe50 [kvm] __se_sys_ioctl+0x7b/0xd0 __x64_sys_ioctl+0x21/0x30 x64_sys_call+0x15d0/0x2e60 do_syscall_64+0x83/0x160 entry_SYSCALL_64_after_hwframe+0x76/0x7e -> #2 (cpu_hotplug_lock){++++}-{0:0}: cpus_read_lock+0x2e/0xb0 static_key_slow_inc+0x16/0x30 kvm_lapic_set_base+0x6a/0x1c0 [kvm] kvm_set_apic_base+0x8f/0xe0 [kvm] kvm_set_msr_common+0x9ae/0xf80 [kvm] vmx_set_msr+0xa54/0xbe0 [kvm_intel] __kvm_set_msr+0xb6/0x1a0 [kvm] kvm_arch_vcpu_ioctl+0xeca/0x10c0 [kvm] kvm_vcpu_ioctl+0x485/0x5b0 [kvm] __se_sys_ioctl+0x7b/0xd0 __x64_sys_ioctl+0x21/0x30 x64_sys_call+0x15d0/0x2e60 do_syscall_64+0x83/0x160 entry_SYSCALL_64_after_hwframe+0x76/0x7e -> #1 (&kvm->srcu){.+.+}-{0:0}: __synchronize_srcu+0x44/0x1a0 synchronize_srcu_expedited+0x21/0x30 kvm_swap_active_memslots+0x110/0x1c0 [kvm] kvm_set_memslot+0x360/0x620 [kvm] __kvm_set_memory_region+0x27b/0x300 [kvm] kvm_vm_ioctl_set_memory_region+0x43/0x60 [kvm] kvm_vm_ioctl+0x295/0x650 [kvm] __se_sys_ioctl+0x7b/0xd0 __x64_sys_ioctl+0x21/0x30 x64_sys_call+0x15d0/0x2e60 do_syscall_64+0x83/0x160 entry_SYSCALL_64_after_hwframe+0x76/0x7e -> #0 (&kvm->slots_lock){+.+.}-{3:3}: __lock_acquire+0x15ef/0x2e30 lock_acquire+0xe0/0x260 __mutex_lock+0x6a/0xb40 mutex_lock_nested+0x1f/0x30 set_nx_huge_pages+0x179/0x1e0 [kvm] param_attr_store+0x93/0x100 module_attr_store+0x22/0x40 sysfs_kf_write+0x81/0xb0 kernfs_fop_write_iter+0x133/0x1d0 vfs_write+0x28d/0x380 ksys_write+0x70/0xe0 __x64_sys_write+0x1f/0x30 x64_sys_call+0x281b/0x2e60 do_syscall_64+0x83/0x160 entry_SYSCALL_64_after_hwframe+0x76/0x7e Cc: Chao Gao <chao.gao@intel.com> Fixes: 0bf50497f03b ("KVM: Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock") Cc: stable@vger.kernel.org Reviewed-by: Kai Huang <kai.huang@intel.com> Acked-by: Kai Huang <kai.huang@intel.com> Tested-by: Farrah Chen <farrah.chen@intel.com> Signed-off-by: Sean Christopherson <seanjc@google.com> --- Documentation/virt/kvm/locking.rst | 32 +++++++++++++++++++++--------- virt/kvm/kvm_main.c | 31 +++++++++++++++-------------- 2 files changed, 39 insertions(+), 24 deletions(-)