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

[Igor Mammedov]

On Mon, 04 Jan 2021 13:54:32 +0100
Vitaly Kuznetsov <vkuznets@redhat.com> 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.
> 
> 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.

for versioning device models (cpu included), we typically set some default in
device's ininfn, and if later on we need to change it to another default
we use compat properties to keep old default to old machine types.

For example using it with CPU look at pc_compat_3_1

> 
> >> >  
> >> > +    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.
there is a lot of way to implement something, in this case point is to
keep it consistent with the way we handle cpu features/properties.
And not to make up new semantics unless there is no other way.

> > 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 :-)
that was just an example

> 
> 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.
ideally it would be a property of device that implements the feature
and machine might enable it depending on its own properties defaults,
but if you change the default behavior of the device model, you do
it in device model and use compat properties infrastructure to keep
old machine types happy.

>