Re: [PATCH 5/5] i386: provide simple 'hyperv=on' option to x86 machine types

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Jan 04, 2021 at 01:54:32PM +0100, Vitaly Kuznetsov wrote:
> Igor Mammedov <imammedo@redhat.com> writes:
> 
> >> >  
> >> > +    /* Hyper-V features enabled with 'hyperv=on' */
> >> > +    x86mc->default_hyperv_features = BIT(HYPERV_FEAT_RELAXED) |
> >> > +        BIT(HYPERV_FEAT_VAPIC) | BIT(HYPERV_FEAT_TIME) |
> >> > +        BIT(HYPERV_FEAT_CRASH) | BIT(HYPERV_FEAT_RESET) |
> >> > +        BIT(HYPERV_FEAT_VPINDEX) | BIT(HYPERV_FEAT_RUNTIME) |
> >> > +        BIT(HYPERV_FEAT_SYNIC) | BIT(HYPERV_FEAT_STIMER) |
> >> > +        BIT(HYPERV_FEAT_FREQUENCIES) | BIT(HYPERV_FEAT_REENLIGHTENMENT) |
> >> > +        BIT(HYPERV_FEAT_TLBFLUSH) | BIT(HYPERV_FEAT_EVMCS) |
> >> > +        BIT(HYPERV_FEAT_IPI) | BIT(HYPERV_FEAT_STIMER_DIRECT);
> > I'd argue that feature bits do not belong to machine code at all.
> > If we have to involve machine at all then it should be a set property/value pairs
> > that machine will set on CPU object (I'm not convinced that doing it
> > from machine code is good idea though).
> >
> 
> These are 'features' and not feature bits. 'Bits' here are just our
> internal (to QEMU) representation of which features are enable and which
> are not, we could've just used booleans instead. These feature, when
> enabled, will result in some CPUID changes (not 1:1) but I don't see how
> it's different from
>   
> " -machine q35,accel=kvm "
> 
> which also results in CPUID changes.

This is a good point, although having accel affect CPUID bits was
also a source of complexity for query-cpu-model-expansion and
other QMP queries.

> 
> The main reason for putting this to x86 machine type is versioning, as
> we go along we will (hopefully) be implementing more and more Hyper-V
> features but we want to provide 'one knob to rule them all' but do it in
> a way that will allow migration. We already have 'hv_passthrough' for
> CPU.

I agree completely that the set of bits needs to be on
MachineClass.  We just need to agree on the external interface.

> 
> >> >  
> >> > +    if (x86ms->hyperv_enabled) {
> >> > +        feat = x86mc->default_hyperv_features;
> >> > +        /* Enlightened VMCS is only available on Intel/VMX */
> >> > +        if (!cpu_has_vmx(&cpu->env)) {
> >> > +            feat &= ~BIT(HYPERV_FEAT_EVMCS);
> >> > +        }
> >> > +
> >> > +        cpu->hyperv_features |= feat;
> > that will ignore features user explicitly doesn't want,
> > ex:
> >  -machine hyperv=on -cpu foo,hv-foo=off
> >
> 
> Existing 'hv_passthrough' mode can also affect the result. Personally, I
> don't see where 'hv-foo=off' is needed outside of debugging and these
> use-cases can probably be covered by explicitly listing required
> features but I'm not against making this work, shouldn't be hard.

I'm all for not wasting time supporting use cases that are not
necessary in practice.  We just need to document the expected
behavior clearly, whatever we decide to do.

> 
> > not sure we would like to introduce such invariant,
> > in normal qom property handling the latest set property should have effect
> > (all other invariants we have in x86 cpu property semantics are comming from legacy handling
> > and I plan to deprecate them (it will affect x86 and sparc cpus) so CPUs will behave like
> > any other QOM object when it come to property handling)
> >  
> > anyways it's confusing a bit to have cpu flags to come from 2 different places
> >
> > -cpu hyperv-use-preset=on,hv-foo=off
> >
> > looks less confusing and will heave expected effect
> >
> 
> Honestly, 'hyperv-use-preset' is confusing even to me :-)
> 
> What if we for a second stop thinking about Hyper-V features being CPU
> features only, e.g. if we want to create Dynamic Memory or PTP or any
> other Hyper-V specific device in a simple way? We'll have to put these
> under machine type.

I agree.  Hyper-V is not just a set of CPU features.

Also, those two approaches are not mutually exclusive.
"-machine hyperv=on" can be implemented internally using
"hyperv-use-preset=on" if necessary.  I don't think it has to,
however.

-- 
Eduardo