Re: Xen Guest Timer (ARM) question

[Ian Campbell <ian.campbell@citrix.com>]

On Sun, 2014-08-31 at 15:01 +0530, embeddev student wrote:

> Q1. Why is guest not given only a virtual timer, What is the need for
> the guest to have a physical timer as well ?

Some guest kernels may not know about the virt timer yet, since it is
new, or they may choose not to use it for some reason (not so likely
since we require paravirt of the timers under Xen today, but eventually
we may want to think about running unmodified OSes on ARM).

It can also occasionally be useful for Paravirtualisation to know both
the real and virtual time (although I can't think of an example off
hand...)

> Q2. In virt_timer_save set_timer is called. If vcpu is going in a
> suspended state what is the requirement of a timer being set ?

This is setting a Xen software timer, not a hardware timer (except that
setting  software timer may trigger the hypervisor timer to be used to
fulfil it).

Since we are descheduling the vcpu we save its virt+phys timer state,
including the comparators, so we can use the hardware for the next vcpu.
The timer is there to make sure we wake up the vcpu when its timer would
have expired and triggered an interrupt.

Ian.