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

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 23 Feb 2021 16:46:50 +0100
Vitaly Kuznetsov <vkuznets@redhat.com> wrote:

> 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
> partition.
> 
> 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
I'd assume it would be hard for libvirt to use 'hv-default' from migration
point of view. It's semi opaque (one can find out what features it sets
indirectly inspecting individual hv_foo features, and mgmt will need to
know about them). If it will mutate when other features [un]set, upper
layers might need to enumerate all these permutations to know which hosts
are compatible or compare host feature sets every time before attempting
migration.

> 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.

we were talking about hv-passthrough, and if host advertises hv-evmcs
QEMU should complain if user disabled features it depends on (
not silently fixing up configuration error).
But the same applies to hv-default.

> 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.

if desired behavior, on Intel host for above config, to start without error
then indeed defaults should not set 'hv-evmcs' if it results in invalid
feature set.