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

[Paolo Bonzini <pbonzini@redhat.com>]

> 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?

Paolo