Re: [PATCH v2 for Xen 4.6 1/4] xen: enabling XL to set per-VCPU parameters of a domain for RTDS scheduler

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Fri, 2015-05-29 at 14:14 +0100, Jan Beulich wrote:
> >>> On 29.05.15 at 15:01, <dario.faggioli@citrix.com> wrote:
> > On Wed, 2015-05-27 at 12:41 +0100, Jan Beulich wrote:
> >> >>> On 26.05.15 at 19:18, <lichong659@gmail.com> wrote:
> >> > On Tue, May 26, 2015 at 4:08 AM, Jan Beulich <JBeulich@suse.com> wrote:
> >> >>>>> On 26.05.15 at 02:05, <lichong659@gmail.com> wrote:
> >> >>> +
> >> >>> +    switch ( op->cmd )
> >> >>> +    {
> >> >>> +    case XEN_DOMCTL_SCHEDOP_getvcpuinfo:
> >> >>> +        spin_lock_irqsave(&prv->lock, flags);
> >> >>> +        list_for_each( iter, &sdom->vcpu )
> >> >>> +        {
> >> >>> +            svc = list_entry(iter, struct rt_vcpu, sdom_elem);
> >> >>> +            vcpuid = svc->vcpu->vcpu_id;
> >> >>> +
> >> >>> +            local_sched.budget = svc->budget / MICROSECS(1);
> >> >>> +            local_sched.period = svc->period / MICROSECS(1);
> >> >>> +            if ( copy_to_guest_offset(op->u.rtds.vcpus, vcpuid,
> >> >>
> >> >> What makes you think that the caller has provided enough slots?
> >> > 
> >> > This code works in a similar way as the one for XEN_SYSCTL_numainfo.
> >> > So if there is no enough slot in guest space, en error code is
> >> > returned.
> >> 
> >> I don't think I saw any place you returned an error here. The
> >> only error being returned is -EFAULT when the copy-out fails.
> >> 
> > Look again at XEN_SYSCTL_numainfo, in particular, at this part:
> > 
> >     if ( ni->num_nodes < num_nodes )
> >     {
> >         ret = -ENOBUFS;
> >         i = num_nodes;
> >     }
> >     else
> >         i = 0
> > 
> > Which, as you'll appreciate if you check from where ni->num_nodes and
> > num_nodes come from, if where the boundary checking we're asking for
> > happens.
> > 
> > Note how the 'i' variable is used in the rest of the block, and you'll
> > see what we mean, and can do something similar.
> 
> I think this was targeted at Chong rather than me (while I was
> listed as To, and Chong only on Cc)?
> 
It was indeed targeted at him. :-)

I replied to your email to re-use and quote the points you made, but did
not think about changing what my MUA does by default when hitting
'Reply'... I never do that, actually, but I now see how it could be a
good idea to do so... I'll try to remember and fit that in my workflow.

> >> >>> +/* Adjust scheduling parameter for the vcpus of a given domain. */
> >> >>> +long sched_adjust_vcpu(
> >> >>> +    struct domain *d,
> >> >>> +    struct xen_domctl_scheduler_vcpu_op *op)
> >> >>> +{
> >> >>> +    long ret;
> >> >>
> >> >> I see no reason for this and the function return type to be "long".
> >> > 
> >> > The reason is stated in the begining.
> >> 
> >> In the beginning of what? I don't see any such, and it would also be
> >> odd for there to be an explanation of why a particular type was chosen.
> >>
> > As Chong he's saying elsewhere, he's moslty following suit. Should we
> > consider a pre-patch making both sched_adjust() and
> > sched_adjust_global() retunr int?
> 
> Perhaps. But the main point here is that people copying existing code
> should sanity check what they copy (so to not spread oddities or even
> bugs).
> 
Agreed, but in this case, he'd have ended up with:

long sched_adjust(...)
int sched_adjust_vcpu(...)
long sched_adjust_global(...)

So I think I see why Chong just went with 'long', that was my point.
Then of course, one could have spotted that it's the two existing ones
that are wrong/suboptimal, but that's more our responsibility than
Chong's fault, IMO (which does not mean that we should not point the
thing out and fix/ask to fix).

In any case, as far as this series is concerned, there should be no need
for a new function like this any longer. For "fixing" _adjust and
_adjust_global, I'll take care of it.

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)

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