On Wed, 2022-11-02 at 23:19 +0000, Sean Christopherson wrote:
> From: Chao Gao <chao.gao@xxxxxxxxx>
>
> Disable CPU hotplug during hardware_enable_all() 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_enable_all() is invoked on another CPU right
>
> the hotplugged CPU will be included in on_each_cpu() and thus get sent
> through hardware_enable_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 | 8 +++++++-
> virt/kvm/kvm_main.c | 10 ++++++++++
> 2 files changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a7b1d916ecb2..a15e54ba0471 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9283,7 +9283,13 @@ static int kvm_x86_check_processor_compatibility(struct kvm_x86_init_ops *ops)
> int cpu = smp_processor_id();
> struct cpuinfo_x86 *c = &cpu_data(cpu);
>
> - WARN_ON(!irqs_disabled());
> + /*
> + * 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(!irqs_disabled() && cpu_active(cpu));
Comment doesn't match with the code?
"KVM should never perform a compatibility check on on offline CPU" should be
something like:
WARN_ON(!cpu_online(cpu));
So, should the comment be something like below?
"KVM compatibility check happens before CPU is marked as active".
>
> 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 fd9e39c85549..4e765ef9f4bd 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -5088,6 +5088,15 @@ 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().
> + *
> + * Disable CPU hotplug to prevent scenarios where KVM sees
> + */
The above sentence is broken.
I think below comment Quoted from Isaku's series should be OK?
/*
* During onlining a CPU, cpu_online_mask is set before
kvm_online_cpu()
* is called. on_each_cpu() between them includes the CPU. As a result,
* hardware_enable_nolock() may get invoked before kvm_online_cpu().
* This would enable hardware virtualization on that cpu without
* compatibility checks, which can potentially crash system or break
* running VMs.
*
* Disable CPU hotplug to prevent this case from happening.
*/
> + cpus_read_lock();
> raw_spin_lock(&kvm_count_lock);
>
> kvm_usage_count++;
> @@ -5102,6 +5111,7 @@ static int hardware_enable_all(void)
> }
>
> raw_spin_unlock(&kvm_count_lock);
> + cpus_read_unlock();
>
> return r;
> }
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