Re: [Xen-devel] linux: {start, stop}_hz_timer() not really affecting periodic timer?

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> 16.01.08 15:37 >>>
>No, no, and no. :-)
>
>start/stop_hz_timer() refer to Linux's own hz ticker. Xen does not deliver
>periodic ticks when a guest is descheduled, so the Xen side of things is
>implicitly handled already. There is no need for start/stop_hz_timer to
>execute hypercalls to enact this.

Okay, okay, I didn't pay attention to this. But then
VCPU_stop_periodic_timer seems a rather academic operation?

>The call to VCPUOP_set_singleshot_timer cannot return -ETIME because the
>kernel does not specify the VCPU_SSHOTTMR_future flag.

I noticed this after pushing the send button. Nevertheless, the whole
construct in stop_hz_timer() seems to assume that it is called with
interrupts disabled, which might be the case now but nothing enforces
xen_safe_halt() to only be called in such contexts... For that reason it
would seem safer to set the flag, check for -ETIME, and avoid
HYPERVISOR_block() altogether in that case.

>All this assume you are looking at linux-2.6.18-xen.hg.

I am, with one eye.

Jan


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