Re: [Qemu-devel] [PATCH 03/10] target-i386: move hyperv_* static globals to CPUState

[Igor Mammedov]

On Tue, 26 Feb 2013 14:06:05 -0300
Eduardo Habkost <address@hidden> wrote:

> On Tue, Feb 26, 2013 at 05:39:02PM +0100, Igor Mammedov wrote:
> [...]
> > > > > > > * I don't expect hv-* to appear on a machine-type compat_props
> > > > > > > array in the near feature.
> > > > > > > * I don't expect people to need to set per-CPU hv-* properties
> > > > > > > on device_add for CPU hotplug.
> > > > > > > 
> > > > > > > So we could keep them as special cases on parse_featurestr(),
> > > > > > > and convert them to per-CPU properties only after we have the
> > > > > > > subclasses and CPU hotplug working.
> > > > > > it won't be a consistent interface, where user who has 
> > > > > > "-cpu XXX,+foo1,+hv_spinlocks,+foo2" on cmd-line
> > > > > > would have to use "device_add XXX,foo1=true,foo2=true" manually
> > > > > > excluding options from device_add, i.e. it propagates special
> > > > > > casing to users as well. And when hv_ are moved to per-CPU
> > > > > > fields, it might break users since they will still exclude some
> > > > > > options.
> > > > > 
> > > > > Won't -cpu/parse_featurestr() simply set global properties? In this
> > > > > case, the common case would be to call "device_add XXX" with no
> > > > > extra options at all, so there's no option to be excluded and no
> > > > > special case to care about.
> > > > That is if global properties will made to 1.5  which I highly doubt
> > > > taking in account how fast patches are reviewed and accepted.
> > > > Otherwise release would be broken. 
> > > 
> > > IMO it _has_ to make 1.5 and is a requirement to make device_add usable
> > > for CPU hotplug. Otherwise we would have to change the behavior of -cpu
> > > + device_add in an incompatible way.
> > if all -cpu features are converted to static properties, we do not have to
> > have global properties working. In absence of 'global properties', user
> > will have to use the same properties at device_add that was specified at
> > -cpu. And introduction of global properties won't regress it, it will
> > just allow to use device_add without features specified on -cpu
> 
> Strictly, we do not have to, but changing -cpu to set global properties
> only later would change the behavior of "-cpu XXX,foo=1,bar=2" followed
> by "device_add XXX" in an incompatible way. So if our long-term plan is
Could you explain how ^^^ it will be incompatible, pls?

> to make "-cpu" change the global-properties/defaults, we need to do it
> since the beginning.
> 
> > > > > > > 
> > > > > > > Considering that -cpu options will be translated to global
> > > > > > > properties, it will be trivial to keep compatibility with
> > > > > > > existing behavior of "-cpu hv_*=..." once we change them from
> > > > > > > static variables to per-CPU fields.
> > > > > > Global properties would just allow not to specify foo1,foo2 on
> > > > > > device_add from hot-plug POV.
> > > > > > 
> > > > > > If this and following patch are to complex we could fallback to
> > > > > > alternative from v6 for hv_* features, which produce the same
> > > > > > external property interface just with different internal approach.
> > > > > 
> > > > > No, v6 exposes completely different (and unexpected) semantics. It
> > > > > exposes a per-CPU property that affects all CPU objects when it gets
> > > > > changed.
> > > > Do you know any guest that will work with mixed CPUs that have and
> > > > doesn't have hv_* set at the same time?
> > > 
> > > I don't know and I don't care. But in either case you are introducing a
> > > per-CPU QOM property for it. If it is a per-CPU QOM property, it just
> > > makes sense that it will affect only the CPU object being changed.
> > > 
> > > What I am proposing is to make it a per-CPU QOM property only after we
> > > make it really per-CPU, so we don't introduce weird externally-visible
> > > property semantics that are likely to change in the near future.
> > Well this patch and next do it per-CPU as you asked, so no weird property
> > semantics is introduced and no special-casing of anything is required.
> 
> Yes, this patch does what I asked for, and I am not really against it.
> :-)
> 
> I am just wondering if we could do these changes later to make review
> and testing easier.
>