Re: [PATCH v5 09/11] x86/domctl: add XEN_DOMCTL_vmtrace_op

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.07.2020 12:38, Julien Grall wrote:
> On 06/07/2020 11:37, Jan Beulich wrote:
>> On 06.07.2020 12:31, Julien Grall wrote:
>>> On 05/07/2020 19:55, Michał Leszczyński wrote:
>>>> +/* XEN_DOMCTL_vmtrace_op: Perform VM tracing related operation */
>>>> +#if defined(__XEN__) || defined(__XEN_TOOLS__)
>>>> +
>>>> +struct xen_domctl_vmtrace_op {
>>>> +    /* IN variable */
>>>> +    uint32_t cmd;
>>>> +/* Enable/disable external vmtrace for given domain */
>>>> +#define XEN_DOMCTL_vmtrace_pt_enable      1
>>>> +#define XEN_DOMCTL_vmtrace_pt_disable     2
>>>> +#define XEN_DOMCTL_vmtrace_pt_get_offset  3
>>>> +    domid_t domain;
>>>
>>> AFAICT, there is a 16-bit implicit padding here and ...
>>>
>>>
>>>> +    uint32_t vcpu;
>>>
>>> ... a 32-bit implicit padding here. I would suggest to make
>>> the two explicit.
>>>
>>> We possibly want to check they are also always zero.
>>
>> Not just possibly imo - we should.
> 
> I wasn't sure given that DOMCTL is not a stable interface.

True; checking padding fields allows assigning meaning to them
without bumping the domctl interface version.

Jan