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

[Ian Campbell <ian.campbell@citrix.com>]

On Tue, 2015-06-30 at 17:54 -0700, Meng Xu wrote:
> Hi Ian,
> 
> 2015-06-30 9:19 GMT-07:00 Ian Campbell <ian.campbell@citrix.com>:
> > On Tue, 2015-06-30 at 11:10 -0500, Chong Li wrote:
> >> On Tue, Jun 30, 2015 at 10:57 AM, Ian Campbell <ian.campbell@citrix.com> wrote:
> >> > On Tue, 2015-06-30 at 10:42 -0500, Chong Li wrote:
> >> >> >> @@ -1554,10 +1564,17 @@ int libxl_sched_credit_params_set(libxl_ctx *ctx, uint32_t poolid,
> >> >> >>  #define LIBXL_DOMAIN_SCHED_PARAM_EXTRATIME_DEFAULT -1
> >> >> >>  #define LIBXL_DOMAIN_SCHED_PARAM_BUDGET_DEFAULT    -1
> >> >> >>
> >> >> >> +/* Per-VCPU parameters*/
> >> >> >> +#define LIBXL_DOMAIN_SCHED_PARAM_VCPU_INDEX_DEFAULT -1
> >> >> >
> >> >> > What is the effect of passing vcpuid == -1 to this interface?
> >> >>
> >> >> I think the right way should be:
> >> >>
> >> >> #define LIBXL_SCHED_PARAM_VCPU_INDEX_DEFAULT -1
> >> >>
> >> >> which is the default value for the vcpuid in libxl_sched_params. Even
> >> >> though libxl_sched_params is used for per-vcpu settings, all the other
> >> >> fields (except vcpuid) would still use
> >> >> LIBXL_DOMAIN_SCHED_PARAM_*_DEFAULT values, just for convenience.
> >> >
> >> > My question is what does the concept of vcpu==-1 mean in this interface?
> >>
> >> We need a default value for vcpuid when initiating libxl_sched_params,
> >> just like other fields (e.g., budget, weight ...) in the struct. But
> >> the default value could be others, maybe "0". I'm not sure.
> >
> > What will you do if you come across a -1 in this field? What are the
> > semantics of the interface WRT this field.
> >
> > 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?

Ian.