Re: [Xen-devel] [PATCH] Bug #327 xentop/libxenstat vcpu fix

[Daniel Stekloff <dsteklof@xxxxxxxxxx>]

On Sunday 16 October 2005 01:38, Keir Fraser wrote:
> On 16 Oct 2005, at 01:11, Daniel Stekloff wrote:
> > Ok.. so xm top/xentop is no longer broken - provided you don't care
> > about out
> > of order vcpus.
>
> This doesn't make sense..... see below.
>
> > xm vcpu-list is broken.
>
> Yes, Ewan is working on a fix for this.
>
> > xm vcpu-enable will only enable vcpus in order.  I can't, for
> > instance, enable
> > vcpu 3 if vcpu 2 isn't enabled. xm vcpu-enable silently fails.
>
> This is currently supposed to work. It looks as though the 'xm
> vcpu-enable/disable' commands are still not 100% robust. :-( Longer
> term, allowing an admin to take down specific vcpus probably isn't all
> that useful, so we may simplify the xm interface.
>
> > Do we no longer have or care about dicontiguous online maps? Or, do
> > the tools
> > just need to catch up with your changes?
>
> Although we don't care about exposing them via vcpu-enable/disable, I
> think we ought to handle them. A user logged into domU can still take
> down non-contiguous VCPUs, even if that functionality isn't exported
> via the xm interface.
>
> > I'm curious because xentop just takes the number of vcpus from
> > domaininfo and
> > calls GETVCPUCONTEXT starting with 0 and going until num-1. I just
> > tried this
> > on a 4 vcpu dom. I disabled vcpus 1 and 2, leaving 0 and 3 active.
> > xentop
> > worked fine but reported those vcpus as 0 and 1. GETVCPUCONTEXT
> > returned the
> > as 0 and 1. Is this the way you want it to work?
>
> That is definitely not how GETVCPUCONTEXT works. It won't renumber
> vcpus under your feet  -- if you ask for vcpu3 then it is vcpu3's
> information that you will get.
>
> One thing to bear in mind: GETVCPUCONTEXT will return info even about
> vcpus that are not currently online. To check for online status you
> need to use GETVCPUINFO and check the online field that is returned.
>
> Perhaps you iterated from 0 to info.n_vcpu-1? The count from
> GETDOMAININFO only counts online vcpus, but doesn;t tell you which
> vcpus are online. We could also add a count 'max_vcpu' or easier: just
> iterate 0 to MAX_VIRT_CPU-1.
>


Sorry, I should have looked at the changes more closely, I didn't realize:

1) getdomaininfo returns number of active cpus

2) getvcpucontext will now return info about vpcus that aren't active.

I can certainly change xentop to iterate over a domain's vcpus bounded by 
MAX_VIRT_CPU and number of active vcpus, calling getvcpuinfo along the way. 
It had been using the info.n_vcpu as a bound. 

Just as a suggestion, it might be nicer for tools to know max_vcpus rather 
than having to iterate over the entire possible list when they want to know 
how many (enabled and disabled) vcpus a domain has. Say, if someone wanted to 
know how may vcpus have been configured rather than checking the config file. 
It also may be more efficient - less GETVCPUINFO calls. 

Thanks for your help.

Dan

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