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

[Juergen Gross <jgross@xxxxxxxx>]

On 19/09/18 19:28, Wei Liu wrote:
> On Tue, Sep 18, 2018 at 02:25:04PM +0100, 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.
>>
> 
> Having the ability to query parameters is a must. Suppose you change
> some settings while the domain is running, in order to re-create domain
> with the same parameter (migration) there must be a way for toolstack to
> query the current settings of that domain. I think most if not all
> information is retrieved from xen using binary interface.
> 
> Furthermore, if the string blob is not stored in xen, and there isn't a
> binary interface for *setting* parameters, toolstack will have to
> translate the retrieved binary information into strings again.
> 
> I'm not picky about formats, but please make get and set interfaces
> symmetric (use the same representation).

Did you look into the patches, especially patch 10? The parameters set
are all stored in domain config via libxl__arch_domain_save_config().

This is skipped only with the user specifying a special flag for
"xl domain-set-parameters".


Juergen

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