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

From: Vitaly Kuznetsov
Subject: Re: [PATCH v4 16/21] i386: track explicit 'hv-*' features enablement/disablement
Date: Tue, 23 Feb 2021 16:46:50 +0100

Igor Mammedov <imammedo@redhat.com> writes:

> On Mon, 22 Feb 2021 11:20:34 +0100
> Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>> 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
> right, if we need to distinguish between explicit and implicit hv-evmcs set by
> hv-passthrough custom parser probably the way to go.
> However do we need actually need to do it?

I think we really need that. See below ...

> I'd treat 'hv-passthrough,-vmx' the same way as 'hv-passthrough,hv-evmcs,-vmx'
> and it applies not only hv-evmcs but other features hv-passthrough might set
> (i.e. if whatever was [un]set by hv-passthrough in combination with other
> features results in invalid config, QEMU shall error out instead of magically
> altering host provided hv-passthrough value).
> something like:
>   'hv-passthrough,-vmx' when hv-passthrough makes hv-evmcs bit set
> should result in
>   error_setg(errp,"'vmx' feature can't be disabled when hv-evmcs is enabled,"
>                  " either enable 'vmx' or disable 'hv-evmcs' along with 
> disabling 'vmx'"
> making host's features set, *magically* mutable, depending on other user 
> provided features
> is a bit confusing. One would never know what hv-passthrough actually means, 
> and if
> enabling/disabling 'random' feature changes it.
> It's cleaner to do just what user asked (whether implicitly or explicitly) 
> and error out
> in case it ends up in nonsense configuration.

I don't seem to agree this is a sane behavior, especially if you replace
'hv-passthrough' with 'hv-default' above. Removing 'vmx' from CPU for
Windows guests is common if you'd want to avoid nested configuration:
even without any Hyper-V guests created, Windows itself is a Hyper-V

So a sane user will do:

'-cpu host,hv-default,vmx=off' 

and on Intel he will get an error, and on AMD he won't. 

So what you're suggesting actually defeats the whole purpose of
'hv-default' as upper-layer tools (think libvirt) will need to know that
Intel configurations for Windows guests are somewhat different. They'll
need to know what 'hv-evmcs' is. We're back to where we've started.

If we are to follow this approach let's just throw away 'hv-evmcs' from
'hv-default' set, it's going to be much cleaner. But again, I don't
really believe it's the right way to go.


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