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

[Greg Kurz <groug@kaod.org>]

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, 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);