Re: [RFC 0/5] Making KVM_GET_ONE_REG/KVM_SET_ONE_REG generic.

[Avi Kivity <avi@redhat.com>]

On 09/01/2012 03:35 PM, Rusty Russell wrote:
> Avi Kivity <avi@redhat.com> writes:
>>> -Capability: basic
>>> +Capability: KVM_CAP_REG_LIST
>>>  Architectures: arm
>>
>> all
> 
> OK, I guess that's to be true in future.  Fixed.
> 
>>>  Type: vcpu ioctl
>>> -Parameters: struct kvm_msr_list (in/out)
>>> +Parameters: struct kvm_reg_list (in/out)
>>>  Returns: 0 on success; -1 on error
>>>  Errors:
>>> -  E2BIG:     the msr index list is too big to fit in the array specified by
>>> -             the user.
>>> +  E2BIG:     the reg index list is too big to fit in the array specified by
>>> +             the user (the number required will be written into n).
>>>  
>>>  struct kvm_msr_list {
>>> -	__u32 nmsrs; /* number of msrs in entries */
>>> -	__u32 indices[0];
>>> +	__u64 n; /* number of registers in reg[] */
>>> +	__u64 reg[0];
>>>  };
>>>  
>>
>> People complain that this interface is hard to use.
>>
>> How about supplying the address of the array (in addition to n) so you
>> don't have to deal with variable sized arrays, and dropping E2BIG in
>> favour of always updating n (n changed to something bigger than you had
>> -> reallocate and rerun)
> 
> We re-write n anyway, *and* return -E2BIG.  Not returning an error is
> asking for trouble.
> 
> Passing an address in a struct is pretty bad, since it involves
> compatibility wrappers.  

Right, some s390 thing.

> I don't think that is what makes the API hard
> to use.

What is it then?  I forgot what the original complaints/complainers were.

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