RE: [Xen-devel] Question about reserving one CPU for the Xen hypervisor in case of vm exit.

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Liang Yang
> Sent: 19 March 2007 21:56
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] Question about reserving one CPU for the 
> Xen hypervisor in case of vm exit.
> 
> Hi,
> 
> My platform has two dual-core processors with VT-x enabled. 
> Suppose I use 
> "xm vcpu-pin" command to set up a fixed mapping between each physical 
> processor/core to virtual cpu (to avoid possible migration).
> 
> I have three domains, one is dom0, the second is domUP and 
> the third is 
> domUF (HVM domain). I give each domain one CPU and reserve one for 
> hypervisor. What I want to do is to always keep one CPU idle 
> (reserving it 
> for VMM), Xen hyperviso can thus always use this idle CPU 
> whenever a "vm 
> exit" happens and the guest HVM domain still has its own CPU 
> to do some 
> overlapping processing (to improve performance).

That will leave you with one CPU sitting there doing absolutely nothing,
as the VMEXIT handling is all done on the CPU that causes the VMEXIT in
the first place. 

The same applies for hypercalls from the PV side. They all happen on the
same CPU that the guest is running on. 

It's a good idea to allow Dom0 to have it's own CPU, but beyond that,
you're better off sharing the three CPU's between your two guests in one
way or another - obviously, you can't give one and a half CPU to a
guest, so you probably will have to give both guests two CPU's to make
efficient use of the system. Or give one CPU to one guest and two to the
other guest. 

--
Mats
> 
> Is this feasible?
> 
> Thanks,
> 
> Liang
> 
> 
> 
> 
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