Re: [Xen-devel] Xen Guest Timer (ARM) question

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 2 Sep 2014, embeddev student wrote:
> On Sun, Aug 31, 2014 at 7:56 PM, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
> >
> > 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.
> >
> Wouldnt there be a problem in this? If two guests timers are
> configured to expire at the same physical time, how would Xen knows it
> needs to inject the interrupt into which Guest. It always injects
> virtual timer interrupt into current which many be G3.

Each Xen timer would expire independently.
I think you are still confusing hardware timers and Xen timers (struct
timer).


> >From a pure academic point of view, and using ARM virtual timers in
> VMs, below is my take on ARM Timer Virtualization
> (a) Guest would be using a HW Virtual Timer Interrupt  and Hypervisor
> should use HW Hypervisor Timer Interrupt.
> (b) Paravirt Timers may not be required as ARM has provided the same
> feature to minimize the context switches to read the TimerCounter
> (c) If a guest is running only then the virtual time be incremented.
> So if two vcpus are there and there is only 1 physical core
> (hypothetically) then the guest time be incremented as long as any
> vcpu is running for that guest.

This is known not to work. Lots of things need to know exactly how much
*real* time has passed. For example think of a device driver that needs
to use timestamps to program the device.

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