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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

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?

*Not* having a language-level construct around (either an enum or a
#define) means that programs can't take advantage of the preprocessor /
type system to catch bugs; someone calling
libxl_domain_param("xptl=off"); won't get a compile-time error, only a
run-time one; someone calling
libxl_domain_param(LIBXL_DOMAIN_PARAM_XPTL, 0) will.

I'm not completely opposed to the "string blob" idea, but it would be
nice if at least for the common case of simple boolean / integer values,
we could avoid having a string blob.

What about

struct parameter {
  int param_number;
  union {
    int val;
    char special[]
  };
}

Or something like that?  That would give flexibility for special cases
like mentioned above, while allowing the common case to avoid special
encoding / decoding &c.

 -George

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