Re: [Xen-devel] domU to dom0 switch question

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 04/02/2010 07:28 PM, George Valzcig wrote:
> I read from the Xen paper dom0 is used for I/O backend driver - you 
> also explain it - thanks. So if some domU wants to I/O, *and only one 
> physical CPU*, domU gets unscheduled - dom0 scheduled - yes?
Yes.  From Xen's perspective, dom0 is just another domain, so it 
schedules all vcpus the same way; if there's contention for pcpus, then 
it needs to context switch between multiple vcpus according to its 
scheduling algorithm.
> My question 1 is, apart from I/O backend driver, is dom0 used for 
> anything else? I assume application running on domU no I/O. Any other 
> reason why domU will require the help of dom0.
Not really.  Once a domain has started, it is independent of dom0 unless 
it specifically makes use of its services.  An hvm domain using a stub 
domain won't make any direct use of dom0 - all its IO requests will be 
proxied via the stub domain.
> My question 2 is, domU requests some service (like page table update) 
> using hypercall. Will this (executing the hypercall and related code) 
> need dom0?
No.  Xen handles all hypercalls directly (though some of them may 
involve another domain).

    J

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