Re: [Xen-devel] what's inside hypercall page?

[Juergen Gross <jgross@xxxxxxxx>]

On 03/03/16 01:56, quizyjones wrote:
>> do_sched_op is self explaining: it is used for scheduling of the vcpu.
>> A vcpu going to idle is using this hypercall. So any interrupt waking
>> the vcpu up will seem to occur very near to the hypercall.
> 
>> do_xen_version is often used as a very fast way to execute the check
>> for pending events in the hypervisor (kind of polling).
> 
>> do_multicall might run for a long time. So the hypervisor returns to
>> the caller from time to time setting IP to the hypercall. The caller
>> has the chance to react to interrupts and will then continue the
>> hypercall.
>>
>>
>> HTH, Juergen
> 
> 
> Thanks for the replying. Does that mean we cannot predict when will
> these two hypercalls finish? I want to set up an interval to monitor the
> instructions (one time monitor per hypercall), so as to reduce the
> performance cost. This requires an accurate prediction of instructions'
> execution so as to avoid missing hypercalls. Is that possible? The main
> problem is the execution of syscall (0x050f), as each hypercall behaves
> different, how can I predict where will it go after the syscall returns?

You can't predict how long a hypercall will run, as this depends on multiple
factors, like the overall load of the host, values of parameters, ...

A hypercall is by it's nature much more complicated than e.g. a simple
arithmetic operation.

What exactly do you want to achieve?


Juergen


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