Re: [PATCH] kvm-vmx: add module parameter to avoid trapping HLT instructions (v3)

[Joerg Roedel <joro@8bytes.org>]

On Fri, Dec 03, 2010 at 05:38:06PM -0600, Anthony Liguori wrote:
> On 12/03/2010 05:32 PM, Joerg Roedel wrote:
>> On Fri, Dec 03, 2010 at 04:39:22PM -0600, Anthony Liguori wrote:
>>    
>>> +	if (yield_on_hlt)
>>> +		min |= CPU_BASED_HLT_EXITING;
>>>      
>> This approach won't work out on AMD because in HLT the CPU may enter
>> C1e. In C1e the local apic timer interupt is not delivered anymore and
>> when this is the current timer in use the cpu may miss timer ticks or
>> never comes out of HLT again. The guest has no chance to work around
>> this as the Linux idle routine does.
>>    
>
> And this doesn't break old software on bare metal?

Yes it does. In fact, this behavior is documented as Erratum 400 for AMD
CPUs. Linux has a workaround for it for quite some time. You can have a
look at the c1e_idle routine for details.
C1e can also be disabled by the OS. But there are BIOSes which re-enable
it in SMI. So there is the chance that it gets re-enabled whithout an
vmexit.

	Joerg