Re: [Xen-devel] [PATCH 00/12] add per-domain and per-cpupool generic parameters

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 09/18/2018 03:57 PM, Juergen Gross wrote:
> On 18/09/18 15:57, George Dunlap wrote:
>> On 09/18/2018 02:36 PM, Juergen Gross wrote:
>>> On 18/09/18 15:25, George Dunlap wrote:
>>>> On 09/18/2018 12:32 PM, Juergen Gross wrote:
>>>>> On 18/09/18 13:20, Jan Beulich wrote:
>>>>>>>>> On 18.09.18 at 13:10, <jgross@xxxxxxxx> wrote:
>>>>>>> On 18/09/18 12:32, Jan Beulich wrote:
>>>>>>>>>>> On 18.09.18 at 08:02, <jgross@xxxxxxxx> wrote:
>>>>>>>>> Instead of using binary hypervisor interfaces for new parameters of
>>>>>>>>> domains or cpupools this patch series adds support for generic text
>>>>>>>>> based parameter parsing.
>>>>>>>>>
>>>>>>>>> Parameters are defined via new macros similar to those of boot
>>>>>>>>> parameters. Parsing of parameter strings is done via the already
>>>>>>>>> existing boot parameter parsing function which is extended a little
>>>>>>>>> bit.
>>>>>>>>>
>>>>>>>>> Parameter settings can either be specified in configuration files of
>>>>>>>>> domains or cpupools, or they can be set via new xl sub-commands.
>>>>>>>>
>>>>>>>> Without having looked at any of the patches yet (not even their
>>>>>>>> descriptions) I'm still wondering what the benefit of textual 
>>>>>>>> parameters
>>>>>>>> really is: Just like "binary" ones, they become part of the public
>>>>>>>> interface, and hence subsequently can't be changed any more or
>>>>>>>> less than the ones we currently have (in particular, anything valid in
>>>>>>>> a guest config file will imo need to remain to be valid and meaningful
>>>>>>>> down the road).
>>>>>>>
>>>>>>> So lets look what would be needed for adding something like the
>>>>>>> per-domain xpti parameter using binary interfaces:
>>>>>>>
>>>>>>> 1 an extension of some domctl interface, maybe bumping of the domctl
>>>>>>>   interface version
>>>>>>> 2 adding the logic to domctl handling
>>>>>>> 3 adding libxc support
>>>>>>> 4 adding libxl support
>>>>>>> 5 adding a new xl sub-command
>>>>>>> 6 adding domain config support
>>>>>>> 7 adding documentation
>>>>>>>
>>>>>>> With my approach only 2 (in a modified form, parameter handling instead
>>>>>>> of domctl, but comparable in the needed effort) and 7 are needed.
>>>>>>>
>>>>>>> So once the framework is in place it is _much_ easier to add new
>>>>>>> features.
>>>>>>
>>>>>> All the above would hold if the individual options were expressed as
>>>>>> e.g. flags in an extensible bit vector.
>>>>>
>>>>> Who would translate the new option into a bit vector? This would be the
>>>>> tools (libxc/libxl/xl), so those need to be modified for each new bit.
>>>>
>>>> A bit vector would only allow on/off; it wouldn't allow you to set
>>>> numeric parameters, for example.
>>>>
>>>> I like the idea of being able to add configuration parameters without
>>>> having a huge amount of boilerplate; and also of being able to backport
>>>> parameters like xpti without having to worry so much about compatibility.
>>>>
>>>> But I'm not a fan of the idea of using a "string blob" to accomplish
>>>> that.  It's convenient for the exact use case you seem to be
>>>> contemplating: having a user add the string into the xl config file, and
>>>> having nobody but the hypervisor interpret it.  But what about tools
>>>> that may want to set that parameter?  Or tools that want to query the
>>>> parameter, or "introspect" on the domain settings or whatever?  Now they
>>>> have to have a bunch of code to generate and parse the string code.
>>>>
>>>> Could we have a reasonably generic structure / union, with a parameter
>>>> number, that we could pass in instead?  Something like:
>>>>
>>>> struct domain_parameter {
>>>>   int param_num;
>>>>   int val;
>>>> }
>>>>
>>>> And then have a list somewhere of string values -> parameter numbers
>>>> that callers could use to translate strings into values?
>>>>
>>>> That way the above list would look like:
>>>>
>>>> 1. Add new parameter in Xen
>>>> 2. Add a string name -> parameter number in a header somewhere
>>>> 3. Add a libxl #define with that parameter number
>>>>
>>>> You'd have to recompile xl against the new header, but you were probably
>>>> going to do that anyway.
>>>
>>> The string variant is much more flexible.
>>>
>>> It is easy possible to e.g. add a per-domain trace parameter to specify
>>> rather complex trace instrumentations. Doing something like that via a
>>> struct based interface is in the best case complicated.
>>
>> ...or, for instance, specifying the runqueue layout of a cpupool (for
>> schedulers like credit2 which allow such things).  Yes, that is true;
>> but probably a very niche case.
>>
>>> Another advantage of the string based variant is that you don't need a
>>> central header. You can define the parameters where you are implementing
>>> them. No need to expand switch statements and headers, just a local
>>> definition and maybe a handler function.
>>
>> I don't see the lack of central header as a big advantage -- how hard is
>> it to add a single line to a list somewhere?
> 
> That's not very hard.
> 
> You need additional entries for connecting the domctl with the parameter
> setting:
> 
> /* central header: */
> #define PARAM_XPTI 13
> 
> /* domctl handling: */
> switch (param) {
> case PARAM_XPTI: ret = do_param_xpti_setting(value);
>     break;
> 
> /* pv-dom header: */
> int do_param_xpti_setting(int val);
> 
> /* pv-dom handler: */
> int do_param_xpti_setting(int val)
> {
> ...
> }
> 
> So you need to touch at least four files in the hypervisor, plus the
> parsing added in xl.
> 
> The string-only variant needs:
> 
> /* pv-dom handler: */
> static int do_param_xpti_setting(...)
> {
> ...
> }
> custom_domain_param("xpti", ...);
> 
> And that's all. See the difference?

I don't think we need the function prototype or a switch statement.

params.h:
#define PARAM_XPTI 13

[later]
   [PARAM_XPTI]="xpti"

pv-dom.c:

static int do_param_xpti_setting(...)
{
...
}
custom_domain_param(XPTI, do_param_xpti_setting);

Multiplexing over the parameter values would be done the same way as
multiplexing over the string values.

Sure it's a tiny bit more convenient not to have to edit params.h; but
having a parameter number, and for most values an integer value, makes
doing things with these programmaticaly in the toolstack a lot easier
and more robust.

Like I said, I'm not 100% opposed to "string blobs" if other people
think it's a good idea; I just think we can do a bit better.

 -George

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