Commit e4aa7f88 authored by Chao Gao's avatar Chao Gao Committed by Paolo Bonzini
Browse files

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 1b6c0168 "KVM: Keep track of which
cpus have virtualization enabled"), but that's an inefficient, convoluted,
and hacky solution.

Signed-off-by: default avatarChao Gao <chao.gao@intel.com>
[sean: split to separate patch, write changelog]
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Message-Id: <20221130230934.1014142-43-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent aaf12a7b

arch/x86/kvm/x86.c

+10 −1
Original line number Diff line number Diff line
@@ -9296,7 +9296,16 @@ 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))

virt/kvm/kvm_main.c

+12 −0
Original line number Diff line number Diff line
@@ -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;

}