Subject: Re: [RFC PATCH 17/17] KVM: PPC: Add an ioctl for userspace to select which platform to emulate

From: Avi Kivity <[email-masked]>
Date: Thu, 30 Jun 2011 18:16:09 +0300

On 06/30/2011 06:04 PM, Alexander Graf wrote:
>> +4.64 KVM_PPC_SET_PLATFORM
>> +
>> +Capability: none
>> +Architectures: powerpc
>> +Type: vm ioctl
>> +Parameters: struct kvm_ppc_set_platform (in)
>> +Returns: 0, or -1 on error
>> +
>> +This is used by userspace to tell KVM what sort of platform it should
>> +emulate.  The return value of the ioctl tells userspace whether the
>> +emulation it is requesting is supported by KVM.
>> +
>> +struct kvm_ppc_set_platform {
>> +    __u16 platform;        /* defines the OS/hypervisor ABI */
>> +    __u16 guest_arch;    /* e.g. decimal 206 for v2.06 */
>> +    __u32 flags;
>
>
> Please add some padding so we can extend it later if necessary.

Regarding that.  There's another option - the ioctl code embeds the 
structure size.  So if we extend the ioctl parsing to pad up (or 
truncate down) from the user's size to our size, and similarly in the 
other direction, we can get away from this ugliness.

Some years ago I posted a generic helper that did this (and also 
kmalloc'ed and kfree'd the data itself), but it wasn't received 
favourably.  Maybe I should try again (and we can possibly use it in kvm 
even if it is rejected for general use, though that's against our 
principles of pushing all generic infrastructure to the wider kernel).

-- 
error compiling committee.c: too many arguments to function