Re: [Xen-devel] Runtime adjustment of hypervisor parameters

[Juergen Gross <jgross@xxxxxxxx>]

On 04/08/17 15:47, Wei Liu wrote:
> On Fri, Aug 04, 2017 at 03:20:09PM +0200, Juergen Gross wrote:
>> Last year Jan posted a patch series to change hypervisor log level
>> thresholds via xl command [1]. This approach was later modified by Wei
>> resulting in patch series [2].
>>
>> I'd like to follow up with another approach being able to do the same,
>> but being much more flexible:
>>
>> Instead of controlling only loglvl I suggest to add a xl command
>>
>> xl xen-param <parameters>
>>
>> which will take a <parameters> string being parsed by the hypervisor
>> the same way it is parsing boot parameters. Allowed parameters are
>> specified in the hypervisor the same way as boot parameters, but with
>> another set of macros (e.g. custom_runtime_param(), ...). Often enough
>> (e.g. in the loglvl case) the definitions could be just the same, while
>> in other cases they might differ a little bit (example: conring_size
>> would require a different handling as at boot time due to race
>> condition handling).
>>
>> Parsing functions could be reused in most cases, they'd just need to
>> lose the __init modifier.
>>
>> What do you think: is this approach sensible, or can I just put it into
>> /dev/null instead of starting with the patches?
>>
> 
> To me this isn't so much about implementation details. It seems that it
> would increase the maintenance burden because now we need to distinguish
> and keep track of the runtime tunable options, which leads to more code
> and documentation. We need to consider the benefit gain first.

The additional amount of code needed would be less than for dedicated
sysctl options for each possible parameter, as existing parsing logic
could be just reused.

Documentation would be limited to the single new xl command and a note
for each parameter in docs/misc/xen-command-line.markdown whether it is
supported for runtime changes.


Juergen

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