Re: [Xen-devel] Save paused cpu ctx

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.09.18 at 16:38, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>>>> All we want to do is to be able to query the state of any VCPU in the
>>>>> valid range of VCPUs assigned to the domain, online or not. We believe
>>>>> being able to query them is reasonable, and the SDM states that they do
>>>>> have a state (whatever it happens to be: the init state, after reset, 
>>>>> etc.).
>>>>
>>>> I didn't know the SDM stated anything about offline vCPU-s. There's
>>>> (according to my way of looking at things) no bare hardware equivalent
>>>> to this state, which means whatever the SDM says is not applicable.
>>>
>>> Please see page 311:
>>>
>>> https://software.intel.com/sites/default/files/managed/a4/60/325384-sdm-vol-
>>>  
> 
>>> 3abcd.pdf
>>>
>>> The section is indeed called "Processor State After Reset" which is
>>> clearly not great for the purposes of this discussion, but the important
>>> part is "Table 9-1. IA-32 and Intel 64 Processor States Following
>>> Power-up, Reset, or INIT", which I believe illustrates the processor
>>> states we were talking about.
>> 
>> I did not question the existence of this description in the manual. What
>> I continue to question is the presence of something talking about
>> _virtual_ CPU state. You pointing me to hardware state descriptions
>> won't change my view that the state of an offline vCPU (note the v!)
>> is simply undefined, and hence querying it makes no sense.
> 
> Fair enough, if we're making this ontological distrinction between
> physical and virtual CPUs, and further state that what applies to one
> does not necessarily apply to the other I can't argue otherwise. Our
> perspective on this was that the latter would be modelled after the former.
> 
> In that case, we'll try to set errno to something specific for the
> caller for the "VCPU offline" case, to at least be able to know for sure
> that we're dealing with that case and not some other error condition
> (such as being out of bounds with the VCPU index).
> 
> Would maybe EBUSY be appropriate?

I'd keep ENOENT for "no such vCPU" and use ENODATA for "vCPU offline".

Jan



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