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

From: Igor Mammedov
Subject: Re: [PATCH v4 16/21] i386: track explicit 'hv-*' features enablement/disablement
Date: Tue, 23 Feb 2021 16:19:48 +0100

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'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.

> '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).g
> Am I missing something?

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