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 10:23, Greg KH wrote:
> On Fri, Jun 18, 2021 at 10:02:57AM +0200, Paolo Bonzini wrote:
>> 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.
> 
> "Traditionally" the first field of a variable length structure like this
> has the size.  So maybe this needs to be:
> 
> struct kvm_stats_header {
> 	__u32 header_size;

Thinking more about it, I slightly prefer id_offset so that we can later 
give a meaning to any bytes after kvm_stats_header and before id_offset.

Adding four unused bytes (for now always zero) is also useful to future 
proof the struct a bit, thus:

struct kvm_stats_header {
	__u32 flags;
	__u32 name_size;
	__u32 num_desc;
	__u32 id_offset;
	__u32 desc_offset;
	__u32 data_offset;
}

(Indeed num_desc is better than count).

> Wait, what is "name_size" here for?

So that you know the full size of the descriptors is (name_size + 
sizeof(kvm_stats_desc) + name_size) * num_desc.  That's the memory you 
allocate and the size that you can then pass to a single pread system 
call starting from offset desc_offset.

There is certainly room for improvement in that the length of id[] and 
name[] can be unified to name_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.
> 
> So it's specified in the _previous_ header?  That feels wrong, shouldn't
> this descriptor define what is in it?

Compared to e.g. PCI where you can do random-access reads from memory or 
configuration space, reading from a file has slightly different 
tradeoffs.  So designing a file format is slightly different compared to 
designing an in-memory format, or a wire protocol.

Paolo