Re: [Qemu-devel] [PATCH RFC 1/1] arm64: add an option to turn on/off vpmu support

[Andrea Bolognani]

On Sat, 2016-08-13 at 01:06 -0500, Wei Huang wrote:
> > > Wouldn't that mean that you'd be unable to use
> > > 
> > >   -cpu foo,pmu=off
> > > 
> > > if CPU model 'foo' doesn't support a PMU? I'd expect that
> > > to work.
> > 
> > The current precedent (has_el3) doesn't work like that: if
> > foo isn't a CPU which can support EL3 then the property doesn't
> > exist, and it's an error to try to set it.
> 
> V1 sent. I tried to follow everyone's advice. See the following:
> 
> * set default pmu=off
> * like el3, add a new feature ARM_FEATURE_HOST_PMU
> * "pmu" property becomes CPU dependent. Only cortex-a53/cortex-a57/host
> under certain mode support this option
> * change struct ARMCPU field name "has_pmu" ==> "has_host_pmu" because
> IMO "has_pmu" is misleading
> 
> BTW answering Andrea's question above: "-cpu foo,pmu=off" won't be
> allowed in this patch if CPU "foo" doesn't support host-backed PMU. QEMU
> will fail to run in this case. Maybe this is what we want?

After discussing this a bit offline, I came to the conclusion
that there isn't a Single Right Way™ to handle this - both my
proposal and what you implemented are reasonable behaviors
one could expect.

On the other hand, what you implemented:

  * matches x86
  * is more strict than what I proposed, so there's room to
    change it later without breaking any existing guest

so I'm happy with it :)

-- 
Andrea Bolognani / Red Hat / Virtualization