Re: [Qemu-devel] [PATCH] rtc: introduce nmi disable bit handler for cmos

[Radim Krcmar <rkrcmar@redhat.com>]

2015-12-15 05:43-0500, Paolo Bonzini:
>> Hi Paolo,
>> 
>> /* for KVM_GET/SET_VCPU_EVENTS */
>> struct kvm_vcpu_events {
>>  ...
>> struct {
>> 		__u8 injected;
>> 		__u8 pending;
>> 		__u8 masked;
>> 		__u8 pad;
>> 	} nmi;
>>  ...
>> 
>> I found that the nmi.masked property does these enable or disable NMI jobs.
>> So, I think we don't need to add a new bit. Right?
> 
> nmi.masked says whether the CPU is accepting the NMIs, and is cleared
> by the next IRET instruction.  This is a different thing; it probably
> shouldn't affect NMI IPIs, and it definitely should remain set until
> cleared via the RTC.  So it should be something like
> 
>     _u8 external_nmi_disabled;
> 
> or similar.
> 
> *However* I found this in the ICH9 datasheet:
> 
>     The ICH9's I/O APIC can only send interrupts due to interrupts which
>     do not include SMI, NMI or INIT. This means that in IA-32/Intel ® 64
>     based platforms, Front Side Bus interrupt message format delivery modes
>     010 (SMI/PMI), 100 (NMI), and 101 (INIT) as indicated in this section,
>     must not be used and is not supported.
> 
> In theory the PIIX4 could deliver such messages, but perhaps we could
> disable them in the KVM IOAPIC.  If we do this, there is no need for a
> change to struct kvm_vcpu_events, because all external NMI sources will
> be in userspace.
> 
> Radim, what do you think?

I looked at the 440fx, piix, and 82083aa(ioapic) datasheets and the
NMI_EN bit doesn't seem to be propagated into the IOAPIC.
The IOAPIC datasheet doesn't mention a thing about NMI masking and PIIX4
generates NMI on SERR# or IOCHK# so it seems that the NMI_EN feature
only changes the behavior of those two ...

I think it's best to do nothing in KVM.

(q35 guests shouldn't configure IOAPIC to send unsupported messages and
 disabling SMI/NMI/INIT in the in-kernel IOAPIC for piix is risky.)