Re: [Xen-devel] Xen Platform QoS design discussion

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/05/2014 08:31, Xu, Dongxiao wrote:
> > -----Original Message-----
> > From: Jan Beulich [mailto:jbeulich@xxxxxxxx]
> > Sent: Thursday, May 29, 2014 3:02 PM
> > To: george.dunlap@xxxxxxxxxxxxx; Xu, Dongxiao
> > Cc: andrew.cooper3@xxxxxxxxxx; Ian.Campbell@xxxxxxxxxx;
> > xen-devel@xxxxxxxxxxxxx
> > Subject: Re: RE: [Xen-devel] Xen Platform QoS design discussion
> >
> > > > > "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx> 05/29/14 2:46 AM >>>
> > > I think Jan's opinion here is similar to what I proposed in the beginning of 
> > > this
> > thread.
> > > The only difference is that, Jan prefers to get the CQM data per-socket and
> > per-domain
> > > with data copying, while I proposed to get the CQM data per-domain for all
> > sockets
> > > that can reduce the amount of hypercalls.
> > I don't think I ever voiced any preference between these two. All I said it
> > depends on
> > prevalent usage models, and to date I don't think I've seen a proper analysis of
> > what
> > the main usage model would be - it all seems guesswork and/or taking random
> > examples.
> >
> > What I did say I'd prefer is to have all this done outside the hypervisor, with 
> > the
> > hypervisor just providing fundamental infrastructure (MSR accesses).
> Okay. If I understand correctly, you prefer to implement a pure MSR access 
> hypercall for one CPU, and put all other CQM things in libxc/libxl layer.
>
> In this case, if libvert/XenAPI is trying to query a domain's cache utilization 
> in the system (say 2 sockets), then it will trigger _two_ such MSR access 
> hypercalls for CPUs in the 2 different sockets.
> If you are okay with this idea, I am going to implement it.
>   
> Thanks,
> Dongxiao
While I can see the use and attraction of a generic MSR access 
hypercalls, using this method for getting QoS data is going to have 
subsantitally higher overhead than even the original domctl suggestion.
I do not believe it will be an effective means of getting large 
quantities of data from ring0 MSRs into dom0 userspace.  This is not to 
say that having a generic MSR interface is a bad thing, but I don't 
think it should be used for this purpose.
~Andrew

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