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

[Greg Kurz <groug@kaod.org>]

Hi Daniel,

On Tue,  8 Dec 2020 10:45:36 -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: "HV", "PR" and, if no kvm-type was set by the user,
> DEFAULT_KVM_TYPE. This allows us 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 what
> happens if the user does not set this option. This information 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>
> ---
> 
> The case I mentioned in the second paragraph is happening when we try to
> execute a pseries guest with '--enable-kvm' using Paolo's patch 
> "vl: make qemu_get_machine_opts static" [1]:
> 
> $ sudo ./ppc64-softmmu/qemu-system-ppc64 -nographic -nodefaults -machine pseries --enable-kvm
> qemu-system-ppc64: Unknown kvm-type specified '' 
> 
> This happens due to the differences between how qemu_opt_get() and
> object_property_get_str() works when there is no 'kvm-type' specified.
> See [2] for more info about the issue found.
> 
> 
> [1] https://lists.gnu.org/archive/html/qemu-devel/2020-12/msg00471.html
> [2] https://lists.gnu.org/archive/html/qemu-devel/2020-12/msg01578.html 
> 
> 
>  hw/ppc/spapr.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index b7e0894019..32d938dc6a 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -3021,9 +3021,10 @@ 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 (!strcmp(vm_type, DEFAULT_KVM_TYPE)) {
>          return 0;
>      }
>  
> @@ -3131,6 +3132,16 @@ static char *spapr_get_kvm_type(Object *obj, Error **errp)
>  {
>      SpaprMachineState *spapr = SPAPR_MACHINE(obj);
>  
> +    /*
> +     * In case the user didn't set 'kvm-type', return DEFAULT_KVM_TYPE
> +     * instead of NULL. This allows us to be more predictable with what
> +     * is expected to happen in spapr_kvm_type(), since we can stop relying
> +     * on receiving a 'NULL' parameter as a valid input there.
> +     */

Returning what is obviously a default value is straightforward enough
that is doesn't need to a comment IMHO.

> +    if (!spapr->kvm_type) {
> +        return g_strdup(DEFAULT_KVM_TYPE);
> +    }
> +
>      return g_strdup(spapr->kvm_type);

Alternatively you could simply do:

    return g_strdup(spapr->kvm_type ? spapr->kvm_type : DEFAULT_KVM_TYPE);

>  }
>  
> @@ -3273,7 +3284,11 @@ static void spapr_instance_init(Object *obj)
>      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 (HV, PR)."

Why not documenting "auto" as a valid option as well ?

While here you could maybe convert HV and PR to lowercase in order to
have a prettier "hv, pr, auto" set of valid values in the help string.
You'd need to convert the related checks in spapr_kvm_type() to still
check for the uppercase versions for compatibility, eg. by using
g_ascii_strcasecmp().

> +                                    " If not specified, defaults to any available KVM"
> +                                    " module loaded in the host. In case both kvm_hv"
> +                                    " and kvm_pr are loaded, kvm_hv takes precedence.");
> +

s/If not specified/If 'auto'/ and mention 'auto' as being the default value.

>      object_property_add_bool(obj, "modern-hotplug-events",
>                              spapr_get_modern_hotplug_events,
>                              spapr_set_modern_hotplug_events);