Re: [RFC PATCH v2 7/7] KVM: KVM: don't break vcpu 'halt' state due to apfs

[Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>]

On 11/02/2010 03:45 PM, Gleb Natapov wrote:

> kvm_event_needs_reinjection() checks for nmi/interrupts that
> need reinjection (not injection).  Those are nmi/interrupts that
> was injected but injection failed for some reason. For nmi, for
> instance, kvm_arch_vcpu_runnable() checks vcpu->arch.nmi_pending,
> but kvm_event_needs_reinjection() checks for vcpu->arch.nmi_injected.
> NMI moves from nmi_pending to nmi_injected when it is injected into vcpu
> for the first time. CPU cannot be halted in this state.
> 

Yeah, nmi is handled like this way, but for interrupt:
If interruption controller is in userspace (!irqchip_in_kernel(v->kvm)),
kvm_arch_vcpu_runnable() checks vcpu->arch.interrupt.pending and
kvm_event_needs_reinjection() also checks vcpu->arch.interrupt.pending.

Consider this case:

- Guest vcpu executes 'HLT'
- wakeup the vcpu and inject interrupt and apfs is completed at this time
- then the vcpu can't handle apf completion and .done list keeps nonempty.  

Can this case happen? Sorry if i missed it again.

>> Also, the pv guest can only handle an apf completion at one time, it can not ensure
>> vcpu->async_pf.done is empty after kvm_check_async_pf_completion()
>>
> In case of PV guest it will be woken up by apf completion by
> kvm_arch_async_page_present() below.

......

>> As i know, exception can not let guest exit halt state, only NMI/interruption can do it, yes? :-)
> On physical HW exception cannot happen while cpu is in halt state, but
> with PV we define what guest can and cannot expect, so when guest
> explicitly enables apf it should be able to handle it during halt.
> 

Ah, i see, thanks your very much.