On Wed, Dec 01 2021 at 06:59, Kevin Tian wrote:
>> From: Paolo Bonzini <paolo.bonzini@xxxxxxxxx>
>> It should fail the first vmptrld instruction. It will result in a few
>> WARN_ONCE and pr_warn_ratelimited (see vmx_insn_failed). For VMX this
>> should be a pretty bad firmware bug, and it has never been reported.
>> KVM did find some undocumented errata but not this one!
>>
>
> or it may be caused by incompatible CPU capabilities, which is currently
> missing a check in kvm_starting_cpu(). So far the compatibility check is
> done only once before registering cpu hotplug state machine:
>
> for_each_online_cpu(cpu) {
> smp_call_function_single(cpu, check_processor_compat, &c, 1);
> if (r < 0)
> goto out_free_2;
> }
>
> r = cpuhp_setup_state_nocalls(CPUHP_AP_KVM_STARTING, "kvm/cpu:starting",
> kvm_starting_cpu, kvm_dying_cpu);
Duh. This is silly _and_ broken.
Using for_each_inline_cpu() without holding cpus_read_lock() is racy
against concurrent hotplug. But even if the locking is added then
nothing prevents a CPU from being plugged _after_ the lock is dropped.
The right solution is to move the hotplug state into the threaded
section as I pointed out and do:
r = cpuhp_setup_state(CPUHP_AP_KVM_STARTING, "kvm/cpu:starting",
kvm_starting_cpu, kvm_dying_cpu);
which will do the right thing automatically. Checking for compatibility
would just be part of the kvm_starting_cpu() callback.
Thanks,
tglx
