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

[Juergen Gross <jgross@xxxxxxxx>]

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.

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.


Juergen

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