Re: [kvm-devel] [RFC][PATCH 3/4] kvm-vmx: KVM_EXIT_DEBUG on #BP exceptions

[Avi Kivity <avi@qumranet.com>]

Hollis Blanchard wrote:
>
>> We mostly try to keep cpu emulation outside userspace.
>>     
>
> Except for this debug stuff? :)
>
>   

Except when it's on the wrong side of the complexity/benefit or 
performance/cleanliness tradeoffs.

>> (of course, that depends on what happens on real hardware.  Is there a 
>> machine check pin?  or does the cpu generate the exception internally?)
>>     
>
> There is a machine check pin, FWIW. It's the chipset (potentially several IO 
> bridges away) that discovers no device decoded the MMIO address, and 
> asynchronously notifies the core.
>
> I'm happy with machine check as an "MMIO failed" flag, so I guess that leaves 
> us with a separate "inject debug event" ioctl.
>   

In that case it's more reasonable to have a machine check ioctl.  Are 
the other chipset-discovered causes of this?  Perhaps on other ppc boards?

The x86 analog is triple faults.  On real hardware, a triple fault 
asserts a processor pin and shuts down the processor.  The motherboard 
detects this and causes a system reset.  So we have a triple fault exit 
reason, and reset generation happens in userspace.

-- 
error compiling committee.c: too many arguments to function