Re: [PATCH v3 5/5] KVM: Do compatibility checks on hotplugged CPUs

[Sean Christopherson]

On Wed, Feb 09, 2022, Chao Gao wrote:
> At init time, KVM does compatibility checks to ensure that all online
> CPUs support hardware virtualization and a common set of features. But
> KVM uses hotplugged CPUs without such compatibility checks. On Intel
> CPUs, this leads to #GP if the hotplugged CPU doesn't support VMX or
> vmentry failure if the hotplugged CPU doesn't meet minimal feature
> requirements.
> 
> Do compatibility checks when onlining a CPU and abort the online process
> if the hotplugged CPU is incompatible with online CPUs.
> 
> CPU hotplug is disabled during hardware_enable_all() to prevent the corner
> case as shown below. A hotplugged CPU marks itself online in
> cpu_online_mask (1) and enables interrupt (2) before invoking callbacks
> registered in ONLINE section (3). So, if hardware_enable_all() is invoked
> on another CPU right after (2), then on_each_cpu() in hardware_enable_all()
> invokes hardware_enable_nolock() on the hotplugged CPU before
> kvm_online_cpu() is called. This makes the CPU escape from compatibility
> checks, which is risky.
> 
> 	start_secondary { ...
> 		set_cpu_online(smp_processor_id(), true); <- 1
> 		...
> 		local_irq_enable();  <- 2
> 		...
> 		cpu_startup_entry(CPUHP_AP_ONLINE_IDLE); <- 3
> 	}
> 
> Keep compatibility checks at KVM init time. It can help to find
> incompatibility issues earlier and refuse to load arch KVM module
> (e.g., kvm-intel).
> 
> Loosen the WARN_ON in kvm_arch_check_processor_compat so that it
> can be invoked from KVM's CPU hotplug callback (i.e., kvm_online_cpu).
> 
> Opportunistically, add a pr_err() for setup_vmcs_config() path in
> vmx_check_processor_compatibility() so that each possible error path has
> its own error message. Convert printk(KERN_ERR ... to pr_err to please
> checkpatch.pl
> 
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> ---

Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>