RE: [PATCH 1/2] x86/hyperv: Fix hyperv_pcpu_input_arg handling when CPUs go online/offline

[Michael Kelley (LINUX)]

From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> Sent: Tuesday, May 23, 2023 1:05 AM
> 
> "Michael Kelley (LINUX)" <mikelley@xxxxxxxxxxxxx> writes:
> 
> > From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> Sent: Monday, May 22, 2023 1:56 AM
> >>
> >> Michael Kelley <mikelley@xxxxxxxxxxxxx> writes:
> >>
> >
> > [snip]
> >
> >> > diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> >> > index 0f1001d..696004a 100644
> >> > --- a/include/linux/cpuhotplug.h
> >> > +++ b/include/linux/cpuhotplug.h
> >> > @@ -201,6 +201,7 @@ enum cpuhp_state {
> >> >  	/* Online section invoked on the hotplugged CPU from the hotplug thread */
> >> >  	CPUHP_AP_ONLINE_IDLE,
> >> >  	CPUHP_AP_KVM_ONLINE,
> >> > +	CPUHP_AP_HYPERV_ONLINE,
> >>
> >> (Cc: KVM)
> >>
> >> I would very much prefer we swap the order with KVM here: hv_cpu_init()
> >> allocates and sets vCPU's VP assist page which is used by KVM on
> >> CPUHP_AP_KVM_ONLINE:
> >>
> >> kvm_online_cpu() -> __hardware_enable_nolock() ->
> >> kvm_arch_hardware_enable() -> vmx_hardware_enable():
> >>
> >>         /*
> >>          * This can happen if we hot-added a CPU but failed to allocate
> >>          * VP assist page for it.
> >>          */
> >> 	if (kvm_is_using_evmcs() && !hv_get_vp_assist_page(cpu))
> >> 		return -EFAULT;
> >>
> >> With the change, this is likely broken.
> >>
> >> FWIF, KVM also needs current vCPU's VP index (also set by hv_cpu_init())
> >> through __kvm_x86_vendor_init() -> set_hv_tscchange_cb() call chain but
> >> this happens upon KVM module load so CPU hotplug ordering should not
> >> matter as this always happens on a CPU which is already online.
> >>
> >> Generally, as 'KVM on Hyper-V' is a supported scenario, doing Hyper-V
> >> init before KVM's (and vice versa on teardown) makes sense.
> >>
> >> >  	CPUHP_AP_SCHED_WAIT_EMPTY,
> >> >  	CPUHP_AP_SMPBOOT_THREADS,
> >> >  	CPUHP_AP_X86_VDSO_VMA_ONLINE,
> >
> > I have no objection to putting CPUHP_AP_HYPERV_ONLINE first.  I did
> > not give any consideration to a possible dependency between the two. :-(
> > But note that in current code, hv_cpu_init() is running on the
> > CPUHP_AP_ONLINE_DYN state, which is also after KVM.  So this patch
> > doesn't change the order w.r.t. KVM and the VP assist page.   Are things
> > already broken for KVM, or is something else happening that makes it
> > work anyway?
> 
> This looks like a currently present bug indeed so I had to refresh my
> memory.
> 
> KVM's CPUHP_AP_KVM_STARTIN is registered with
> cpuhp_setup_state_nocalls() which means that kvm_starting_cpu() is not
> called for all currently present CPUs. Moreover, kvm_init() is called
> when KVM vendor module (e.g. kvm_intel) is loaded and as KVM is normally
> built as module, this happens much later than Hyper-V's
> hyperv_init(). vmx_hardware_enable() is actually called from
> hardware_enable_all() which happens when the first KVM VM is created.
> 
> This all changes when a CPU is hotplugged. The order CPUHP_AP_* from
> cpuhp_state is respected and KVM's kvm_starting_cpu() is called _before_
> Hyper-V's hv_cpu_init() even before your patch. We don't see the bug
> just because Hyper-V doesn't(?) support CPU hotplug. Just sending a CPU
> offline with e.g. "echo 0 > /sys/devices/system/cpu/cpuX/online" is not
> the same as once allocated, VP assist page persists for all non-root
> Hyper-V partitions. I don't know if KVM is supported for Hyper-V root
> partitions but in case it is, we may have a problem.
> 
> Let's put CPUHP_AP_HYPERV_ONLINE before KVM's CPUHP_AP_KVM_ONLINE
> explicitly so CPU hotplug scenario is handled correctly, even if current
> Hyper-V versions don't support it.
> 

Will do.  I'll send a v2 with the change.

Michael