Re: [PATCH v4 16/21] i386: track explicit 'hv-*' features enablement/disablement

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Vitaly Kuznetsov <vkuznets@redhat.com> writes:

> Igor Mammedov <imammedo@redhat.com> writes:
>
>>> 
>>> We need to distinguish because that would be sane.
>>> 
>>> Enlightened VMCS is an extension to VMX, it can't be used without
>>> it. Genuine Hyper-V doesn't have a knob for enabling and disabling it,
>> ...
>>> That bein said, if
>>> guest CPU lacks VMX it is counter-productive to expose EVMCS. However,
>>> there is a problem with explicit enablement: what should
>>> 
>>> 'hv-passthrough,hv-evmcs' option do? Just silently drop EVMCS? Doesn't
>>> sound sane to me.
>> based on above I'd error out is user asks for unsupported option
>> i.e. no VMX -> no hv-evmcs - if explicitly asked -> error out
>
> That's what I keep telling you but you don't seem to listen. 'Scratch
> CPU' can't possibly help with this use-case because when you parse 
>
> 'hv-passthrough,hv-evmcs,vmx=off' you
>
> 1) "hv-passthrough" -> set EVMCS bit to '1' as it is supported by the
> host.
>
> 2) 'hv-evmcs' -> keep EVMCS bit '1'
>
> 3) 'vmx=off' -> you have no idea where EVMCS bit came from.
>
> We have to remember which options were aquired from the host and which
> were set explicitly by the user. 

Igor,

could you please comment on the above? In case my line of thought is
correct, and it is impossible to distinguish between e.g.

'hv-passthrough,hv-evmcs,-vmx'
and
'hv-passthrough,-vmx'

without a custom parser (written just exactly the way I did in this
version, for example) regardless of when 'hv-passthrough' is
expanded. E.g. we have the exact same problem with
'hv-default,hv-evmcs,-vmx'. I that case I see no point in discussing
'scratch CPUs' idea at this point because it is not going to change
anything at all ('hv_features_on' will stay, custom parsers will stay).

Am I missing something?

-- 
Vitaly