Re: [PATCH v2 1/1] spapr.c: set a 'kvm-type' default value instead of relying on NULL

[Daniel Henrique Barboza]

On 12/10/20 11:14 AM, Greg Kurz wrote:
> On Thu, 10 Dec 2020 15:01:20 +0100
> Greg Kurz <groug@kaod.org> wrote:
>
> > On Thu, 10 Dec 2020 10:07:21 -0300
> > Daniel Henrique Barboza <danielhb413@gmail.com> wrote:
> >
> > > spapr_kvm_type() is considering 'vm_type=NULL' as a valid input, where
> > > the function returns 0. This is relying on the current QEMU machine
> > > options handling logic, where the absence of the 'kvm-type' option
> > > will be reflected as 'vm_type=NULL' in this function.
> > >
> > > This is not robust, and will break if QEMU options code decides to propagate
> > > something else in the case mentioned above (e.g. an empty string instead
> > > of NULL).
> > >
> > > Let's avoid this entirely by setting a non-NULL default value in case of
> > > no user input for 'kvm-type'. spapr_kvm_type() was changed to handle 3 fixed
> > > values of kvm-type: "auto", "hv", and "pr", with "auto" being the default
> > > if no kvm-type was set by the user. This allows us to always be predictable
> > > regardless of any enhancements/changes made in QEMU options mechanics.
> > >
> > > While we're at it, let's also document in 'kvm-type' description the
> > > already existing default mode, now named 'auto'. The information provided
> > > about it is based on how the pseries kernel handles the KVM_CREATE_VM
> > > ioctl(), where the default value '0' makes the kernel choose an available
> > > KVM module to use, giving precedence to kvm_hv. This logic is described in
> > > the kernel source file arch/powerpc/kvm/powerpc.c, function kvm_arch_init_vm().
> > >
> > > Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> > > ---
> > >   hw/ppc/spapr.c | 15 ++++++++++-----
> > >   1 file changed, 10 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > index b7e0894019..f097f4ea30 100644
> > > --- a/hw/ppc/spapr.c
> > > +++ b/hw/ppc/spapr.c
> > > @@ -3021,17 +3021,18 @@ static void spapr_machine_init(MachineState *machine)
> > >       qemu_cond_init(&spapr->fwnmi_machine_check_interlock_cond);
> > >   }
> > >   
> > > +#define DEFAULT_KVM_TYPE "auto"
> > >   static int spapr_kvm_type(MachineState *machine, const char *vm_type)
> > >   {
> > > -    if (!vm_type) {
> > > +    if (!g_ascii_strcasecmp(vm_type, DEFAULT_KVM_TYPE)) {
> >
> > strcmp() would be better here : we don't want to support an
> > already existing "AUTO" value like for the other ones.
>
> And you should also keep the check on vm_type since it is NULL
> if the kvm-type wasn't passed to the command line:
>
> qemu-system-ppc64: GLib: g_ascii_strcasecmp: assertion 's1 != NULL' failed


Hm, I tested this case and didn't hit that. Perhaphs because I am testing this
on top of Paolo's patches instead of master.

I'll keep the NULL check as a fallback to 'auto' just to be safe then.


DHB

> > And, of course, add a comment to explain why the two other
> > ones are handled differently.
> >
> > >           return 0;
> > >       }
> > >   
> > > -    if (!strcmp(vm_type, "HV")) {
> > > +    if (!g_ascii_strcasecmp(vm_type, "hv")) {
> > >           return 1;
> > >       }
> > >   
> > > -    if (!strcmp(vm_type, "PR")) {
> > > +    if (!g_ascii_strcasecmp(vm_type, "pr")) {
> > >           return 2;
> > >       }
> > >   
> > > @@ -3131,7 +3132,7 @@ static char *spapr_get_kvm_type(Object *obj, Error **errp)
> > >   {
> > >       SpaprMachineState *spapr = SPAPR_MACHINE(obj);
> > >   
> > > -    return g_strdup(spapr->kvm_type);
> > > +    return g_strdup(spapr->kvm_type ? spapr->kvm_type : DEFAULT_KVM_TYPE);
> >
> > As in Paolo's diff, it seems better to simply initialize spapr->kvm_type ...
> >
> > >   }
> > >   
> > >   static void spapr_set_kvm_type(Object *obj, const char *value, Error **errp)
> > > @@ -3273,7 +3274,11 @@ static void spapr_instance_init(Object *obj)
> >
> > ... here.
> >
> >      spapr->kvm_type = g_strdup(DEFAULT_KVM_TYPE);
> >
> > >       object_property_add_str(obj, "kvm-type",
> > >                               spapr_get_kvm_type, spapr_set_kvm_type);
> > >       object_property_set_description(obj, "kvm-type",
> > > -                                    "Specifies the KVM virtualization mode (HV, 
> > > PR)");
> > > +                                    "Specifies the KVM virtualization mode 
> > > (auto,"
> > > +                                    " hv, pr). Defaults to 'auto'. This mode will 
> > > use"
> > > +                                    " any available KVM module loaded in the 
> > > host,"
> > > +                                    " where kvm_hv takes precedence if both kvm_hv 
> > > and"
> > > +                                    " kvm_pr are loaded.");
> > >       object_property_add_bool(obj, "modern-hotplug-events",
> > >                               spapr_get_modern_hotplug_events,
> > >                               spapr_set_modern_hotplug_events);