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

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;
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 
The only difference is that, Jan prefers to get the CQM data per-socket and
with data copying, while I proposed to get the CQM data per-domain for all
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
the main usage model would be - it all seems guesswork and/or taking random

What I did say I'd prefer is to have all this done outside the hypervisor, with 
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.

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.


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