Re: [Qemu-devel] [PATCH 2/2] vl: Print CPU help after we've registered the CPU accelerators

[David Gibson]

On Mon, Mar 06, 2017 at 08:47:52AM -0300, Eduardo Habkost wrote:
> On Mon, Mar 06, 2017 at 10:06:27AM +1100, David Gibson wrote:
> > On Fri, Mar 03, 2017 at 11:58:07AM -0300, Eduardo Habkost wrote:
> > > On Tue, Jan 31, 2017 at 02:11:59PM +0100, Thomas Huth wrote:
> > > > When running with KVM on POWER, we register some CPU types during
> > > > the initialization function of the ppc64 KVM code (which unfortunately
> > > > also can not be done via a type_init() like it is done on x86).
> > > 
> > > Can you elaborate why it can't be done via type_init()? If the
> > > QOM type hierarchy depends on any runtime data unavailable at
> > > type_init(), we should fix that.
> > 
> > Hmm.. how?  This is specifically for the special 'host' cpu in the
> > case of KVM.  We can't use a static configuration here, because there
> > are things on the host that could limit what features of the CPU are
> > available for guest use.  So, we need KVM to be initialized in order
> > to query that information.
> 
> There's nothing saying you need to query that information before
> type_register() or during class_init, does it? The behavior of
> TYPE_HOST_POWERPC_CPU after object_new() is called can be as
> dynamic as you want it to, but the QOM type hierarchy is supposed
> to be static.

So, the thing is we have some properties that logically belong in the
CPU class, rather than instance, and that's correct for all TCG CPUs,
but not for the KVM host CPU.  It seems nasty to have to force all
those things into the instance just because of KVM.

> Registering QOM types outside type_init() is only causing us
> problems and we should stop doing that.
> 
> > 
> > > >                                                                 So to
> > > > be able to see these updates in the CPU help text, the code that calls
> > > > list_cpus() has to be run after configure_accelerator(). This move 
> > > > should
> > > > be fine since the "cpu_model" variable is also never used before the 
> > > > call
> > > > to configure_accelerator(), and thus there should not be any unwanted
> > > > side effects in the code before configure_accelerator() if the user
> > > > started QEMU with "-cpu ?" or "-cpu help".
> > > > 
> > > > Signed-off-by: Thomas Huth <address@hidden>
> > > 
> > > I am not convinced that the output of "-cpu help" and
> > > "-cpu help -machine accel=kvm" should look different. Do you have
> > > an example of what exactly is wrong with the output currently?
> > > 
> > > I actually believe list_cpus() needs to be called _earlier_, not
> > > later. Otherwise we won't be able to fix this bug:
> > > 
> > >   $ qemu-system-arm -cpu help
> > >   qemu-system-arm:/usr/local/etc/qemu/qemu.conf:1: No machine specified, 
> > > and there is no default
> > >   Use -machine help to list supported machines
> > >   $ 
> > > 
> > > > ---
> > > >  vl.c | 10 +++++-----
> > > >  1 file changed, 5 insertions(+), 5 deletions(-)
> > > > 
> > > > diff --git a/vl.c b/vl.c
> > > > index 0b72b12..315c5c3 100644
> > > > --- a/vl.c
> > > > +++ b/vl.c
> > > > @@ -4055,11 +4055,6 @@ int main(int argc, char **argv, char **envp)
> > > >          qemu_set_hw_version(machine_class->hw_version);
> > > >      }
> > > >  
> > > > -    if (cpu_model && is_help_option(cpu_model)) {
> > > > -        list_cpus(stdout, &fprintf, cpu_model);
> > > > -        exit(0);
> > > > -    }
> > > > -
> > > >      if (!trace_init_backends()) {
> > > >          exit(1);
> > > >      }
> > > > @@ -4298,6 +4293,11 @@ int main(int argc, char **argv, char **envp)
> > > >  
> > > >      configure_accelerator(current_machine);
> > > >  
> > > > +    if (cpu_model && is_help_option(cpu_model)) {
> > > > +        list_cpus(stdout, &fprintf, cpu_model);
> > > > +        exit(0);
> > > > +    }
> > > > +
> > > >      if (qtest_chrdev) {
> > > >          qtest_init(qtest_chrdev, qtest_log, &error_fatal);
> > > >      }
> > > 
> > 
> 
> 
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

signature.asc
Description: PGP signature