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Re: [Xen-devel] Cache Allocation Technology(CAT) design for XEN


Thanks for posting this - it's very useful.  I have a couple of
questions about the interface design. 

At 20:27 +0800 on 12 Dec (1418412477), Chao Peng wrote:
> Design Overview
> When enforcing cache allocation for VMs, the minimum granularity is
> defined as the domain. All Virtual CPUs ("VCPUs") of a domain have the
> same COS, and therefore, correspond to the same CBM. COS is used only in
> hypervisor and is transparent to tool stack/user. System administrator
> can specify the initial CBM for each domain or change it at runtime using 
> tool stack. Hypervisor then choses a free COS to associate it with that
> CBM or find a existed COS which has the same CBM.

What happens if there is no existing COS that matches, and all COSes
are in use?  Does Xen return an error?  Or try to change COS->CMB
mappings during context switches?

> - VCPU Schedule
> When VCPU is scheduled on the physical CPU ("PCPU"), its COS value is
> then written to MSR (IA32_PQR_ASSOC) of PCPU to notify hardware to use 
> the new COS. The cache allocation is then enforced by hardware.
> - Multi-Socket
> In multi-socket environment, each VCPU may be scheduled on different
> sockets. The hardware CAT ability(such as maximum supported COS and length
> of CBM) maybe different among sockets. For such system, per-socket COS/CBM
> configuration of a domain is specified. Hypervisor then use this per-socket
> CBM information for VCPU schedule.

Is it OK to assume that in the common case all CPUs have the same CAT
capabilities?  Then Xen can just report the smallest set of
capabilities of any socket in the system, and the toolstack doesn't
have to mess about with per-socket settings.

I guess you can add that syntactic sugar on the tools if you want and
leave the more powerful hypecall interface in case it's useful. :)


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