Re: [Xen-devel] [PATCH v3 for Xen 4.6 3/4] libxl: enable per-VCPU parameter settings for RTDS scheduler

[Chong Li <lichong659@xxxxxxxxx>]

On Wed, Jul 1, 2015 at 7:50 AM, Dario Faggioli
<dario.faggioli@xxxxxxxxxx> wrote:
> On Wed, 2015-07-01 at 09:48 +0100, Ian Campbell wrote:
>> On Tue, 2015-06-30 at 17:54 -0700, Meng Xu wrote:
>> > 2015-06-30 9:19 GMT-07:00 Ian Campbell <ian.campbell@xxxxxxxxxx>:
>> > > Note that this field is not the same as the others in this struct, it is
>> > > in effect part of the "key" while the others are the "values".
>> > >
>> >
>> > vcpuid in the libxl is not the key but the value.
>> >
>> > When it is passed into hypervisor, vcpuid acts as the key to identify
>> > which VCPU. vcpuid should be be negative when it is passed to
>> > hypervisor.
>> >
>> > Do you have any concerns about assigning the initial value of vcpuid as -1?
>>
>> libxl needs to do _something_ if it is passed a vcpuid=-1. What is that
>> something?
>>
> Exactly.
>
> Meng, Chong, maybe what you are missing from Ian's point is the fact
> that libxl is a library on top of which to build more advanced
> toolstack, it is not just something that the xl command line tool links
> to.
>
> Therefore, it has to work even when it is not xl that makes the API
> calls. Actually, I better say that --not only it has to work-- it has to
> be easy and comfortable to use from high level components different than
> xl.
>
> The outcome of this reasoning is that, you can't just assume that a
> default value does not matter much, because every code path in _xl_
> overrides it. Everything that is in libxl must make sense in libxl
> per-se, even without bringing xl into the picture.
>
> So, in this case, you need to think whether it is convenient
> _in_general_, not only wrt xl, to have -1 as default value for vcpuid
> and, if yes, what happens if such default is not overridden and libxl
> gets to deal with it!
>
> Hope this helps... If this was not the issue, sorry for the noise. :-)

I did some sanity check about the vcpuid in hypervisor. Maybe that's
not enough. I'll add other check on vcpuid in libxl
(sched_rtds_vcpu_get/set) to ensure vcpuid has a valid value.

Chong
>
> Regards,
> Dario
>
> --
> <<This happens because I choose it to happen!>> (Raistlin Majere)
> -----------------------------------------------------------------
> Dario Faggioli, Ph.D, http://about.me/dario.faggioli
> Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)



-- 
Chong Li
Department of Computer Science and Engineering
Washington University in St.louis

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