Re: what's inside hypercall page?

[Juergen Gross <jgross@suse.com>]

On 02/03/16 15:29, quizyjones wrote:
> After step by step monitoring, I get the following statistics about
> hypercall entries:
> 
> numbers | hypercalls | executed bytes (offset to hypercall entry)
>    7755 24: 0 1 3 8 a c d
>    6374 23: 0 1 3 4 9
>    3281 25: 0 1 3 8 a c d
>    2979 13: 0 1 3 8 a c d
>    2475 17: 0 1 3 8
>    2253 17: a c d
>     749 3: 0 1 3 8 a c d
>     655 23: 0 1 3 4 9 0 1 3 4 9
>     640 29: 0 1 3 8
>     636 29: a c d
>     445 23: 0 1 3 4 9 0 1 3 4 9 0 1 3 4 9
>     433 23: 0 1 3 4 9 0 1 3 4 9 0 1 3 4 9 0 1 3 4 9
>     414 24: 0 1 3 8 a c d 0 1 3 8 a c d
>     274 13: *0 1 3 8 8 a c d*
>     129 17: d
>     125 17: a c
>     112 29: a c d 0 1 3 8
>     112 17: c d
>     105 17: a
>      73 24: 0 1 3 8 a c d 0 1 3 8 a c d 0 1 3 8 a c d
>      67 17: 0
>      59 17: 8 a c d
>      54 17: 0 1 3
>      53 17: 0 1
>      50 17: 1 3 8 a c d
>      46 17: 3 8 a c d
>      21 3: 0 1 3 8 a c d 0 1 3 8 a c d
>       8 33: 0 1 3 8 a c d
>       7 17: 1 3
>       6 13: 0 1 3 8 8 8 a c d
>       5 29: d
>       5 23: 0 1 3 4 9 0 1 3 4 9 0 1 3 4 9 0 1 3 4 9 0 1 3 4 9
>       4 29: a c
>       4 17: 3
>       3 17: 8 a
>       3 17: 8
>       3 17: 3 8
>       3 17: 1 3 8 a c
>       3 17: 1
>       2 29: 0 1 3 8 a c d
>       2 17: 3 8 a
>       2 17: 1 3 8 a
>       2 17: 1 3 8
>       1 29: c
>       1 29: a
>       1 29: 3 8 a c d
>       1 29: 1 3 8 a c d
>       1 29: 0 1
>       1 29: 0
>       1 17: 3 8 a c
> 
> From the above we can see that hypercall #17 and #29 are very irregular,
> with various combination occurs. Other hypercalls basically obey to the
> sequence of "0 1 3 8 a c d" which conforms to the content in
> hypercall_page_initialise function. HYPERCALL_iret is a special one as
> explained in the function, but it also conforms to its sequence of "0 1
> 3 4 9". So why would #17(do_xen_version) and #29(do_sched_op) performs

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).

> irregular? They seem to be easily interrupted at any place of the
> hypercall entry. Besides, there is also some abnormals for
> #13(do_multicall) shown in bold.

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

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