Re: [PATCH v11 2/7] KVM: stats: Add fd-based API to read binary stats data

[Paolo Bonzini <bonzini@gnu.org>]

On 18/06/21 09:00, Greg KH wrote:
>> +struct kvm_stats_header {
>> +	__u32 name_size;
>> +	__u32 count;
>> +	__u32 desc_offset;
>> +	__u32 data_offset;
>> +	char id[];
>> +};
> 
> You mentioned before that the size of this really is the size of the
> structure + KVM_STATS_ID_MAXLEN, right?  Or is it - KVM_STATS_ID_MAXLEN?
> 
> If so, why not put that value explicitly in:
> 	char id[THE_REST_OF_THE_HEADER_SPACE];
> 
> As this is not a variable header size at all, and you can not change it
> going forward, so the variable length array here feels disingenuous.

It can change; the header goes up to desc_offset.  Let's rename 
desc_offset to header_size.

>> +struct kvm_stats_desc {
>> +	__u32 flags;
>> +	__s16 exponent;
>> +	__u16 size;
>> +	__u32 offset;
>> +	__u32 unused;
>> +	char name[];
>> +};
> 
> What is the max length of name?

It's name_size in the header.

> Why aren't these structures defined here in kerneldoc so that we can
> understand them better?  Putting them in a .rst file guarantees they
> will get out of sync, and you can always directly import the kerneldoc
> into the .rst file.

This is a problem in general with Documentation/virt/kvm/api.rst.  The 
file is organized to match the kerneldoc structs to the ioctl that they 
are used for, and sometimes a ioctl includes different structs for each 
architecture.

It is probably possible to do it using :identifiers: and/or :doc:, but 
it would require running scripts/kernel-doc on the uAPI headers dozens 
of times.  That is quite expensive at 0.3s each run, but that's what you 
get with Perl (gcc -fsyntax-only is 20 times faster).

Paolo