Re: [PATCH 09/12] xen/hypfs: add support for id-based dynamic directories

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.11.2020 15:38, Jürgen Groß wrote:
> On 17.11.20 14:33, Jan Beulich wrote:
>> On 26.10.2020 10:13, Juergen Gross wrote:
>>> +static struct hypfs_entry *hypfs_dyndir_findentry(struct hypfs_entry_dir 
>>> *dir,
>>> +                                                  const char *name,
>>> +                                                  unsigned int name_len)
>>> +{
>>> +    struct hypfs_dyndir_id *data;
>>> +
>>> +    data = hypfs_get_dyndata();
>>> +    if ( !data )
>>> +        return ERR_PTR(-ENOENT);
>>> +
>>> +    /* Use template with original findentry function. */
>>> +    return data->template->e.funcs->findentry(data->template, name, 
>>> name_len);
>>
>> Why does this pass the address of the template? If it truly is
>> (just) a template, then its dirlist ought to be empty at all
>> times? If otoh the "template" indeed gets used as a node in the
>> tree then perhaps it wants naming differently? "Stem" would come
>> to mind, but likely there are better alternatives. I've also
>> considered the German "Statthalter", but its English translations
>> don't seem reasonable to me here. And "placeholder" has kind of a
>> negative touch. (Also in this case some of my "const?" remarks
>> may be irrelevant.)
> 
> It is basically a template tree.
> 
> In the current use case (cpupool/<id>/sched-gran) the template is
> <id> with the leaf "sched-gran" which is the template for the per
> cpupool incarnation.

I can see sched-gran being a template, albeit even that will get
dirtied as soon as a second leaf appears, as then these entries
again need linking together. I think anything called template
should be strictly r/o.

It's also not clear to me how adding a 2nd level in the hierarchy
would end up working: <component>/<id1>/<id2>/<leaf>. How would
<leaf> know all the higher level IDs it's associated with? While
I don't think this needs making work right away, the underlying
model at least shouldn't prohibit it.

Jan