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

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 4 Jan 2021 13:29:06 -0500
Eduardo Habkost <ehabkost@redhat.com> wrote:

> 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.

why was, it's still a headache (mutating CPU models depending on accelerator)

> 
> > 
> > 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.
That's where I disagree,
let me exaggerate for demo purpose:
 - let's move all CPU models feature defaults to MachineClass and forget about compat properties
    since in that case we can opencode changes in machine_class_init

It's rather hard code integration between device models, which we try
to avoid and still refactoring QEMU code to get rid of it.
(sure it works until it's not and someone else need to rewrite half of QEMU
to accomplish it's own task because we mixed things together)

> 
> >   
> > >> >  
> > >> > +    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.

documenting is good, but if it adds new semantics to how CPU features are handled
users up the stack will need code it up as well and juggle with
 -machine + -cpu + -device cpu-foo
not to mention poor developers who will have to figure out why we do
set CPU properties in multiple different ways.

however if we add it as CPU properties that behave the same way as other
properties, all mgmt has to do is expose new property to user for usage.

it even more true when building machine from QMP interface would be available,
where we would want '-device foo' more or less the same way instead of
special casing some of them, i.e. I'd rather have one device to configure,
instead of doing it in multiple places. It's not possible in reality
but for new code we should try to minimize split brain issues.

> >   
> > > 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.
me too,
however in this case we are talking about a set of cpu features,
if there is no way to implement it as cpu properties + compat properties
and requires opencodding it within machine code it might be fine
but I fail to see a very good reason for doing that at this momment.

> 
> 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.