[PATCH] clocksource/drivers/arm_arch_timer: export arch_timer_get_rate

[Chanho Park]

Hi Marc,

On Tue, Jan 12, 2021 at 11:45 PM Marc Zyngier <maz at kernel.org> wrote:
>
> On 2021-01-12 13:39, Chanho Park wrote:
> > Hi,
> >
> >> On Tue, Jan 12, 2021 at 10:31:40AM +0900, Chanho Park wrote:
> >> > This patch adds to export arch_timer_get_rate function for calculating
> >> > absolute timestamp which is based on arch timer like below.
> >> > arch_timer_read_counter was already exported but arch_timer_get_rate
> >> > wasn't. Thus, this patch tries to export this to use this function from
> >> > loadable kernel module.
> >>
> >> Can you please explain /where/ this would be used? i.e. which module?
> >>
> >> Generally we try to avoid drivers depending on the specific
> >> clocksource,
> >> so I think there needs to be stronger rationale for exposing this.
> >
> > I need a system-wide timestamp which can be available from bootloader
> > and kernel stages including virtual machines.
> > Actually, it's necessary to record a timestamp of each log message for
> > system-wide debugging on type-1 hypervisor.
> > RTC can be used for this purpose but we should make it to hypervisor
> > awareness.
> > |---------------|-------------------------|
> >  Bootloader        VM1 (Guest)
> >                    |-------------------------|
> >                           VM2 (Guest)
> >
> > So, the easiest way is using the arm architect timer's timestamp
> > because it's already supported on each VM by the hypervisor.
>
> This doesn't make much sense. The hypervisor and the VMs are
> independent software entities, and they don't use symbols from
> each other.

I meant that each VM needs to be synchronized by the ARM arch timer's
timestamp not the symbol itself.
The symbol is necessary to calculate the system-wide time by the
timestamp(counter) value.

The counter of the arm architect timer will be increasing from the bootloader.
The hypervisor will not reset the counter and each VMs as well. So, we
can use this timestamp for comparing _real_ time :)

>
> So this symbol is probably used by a module *inside* the VMs,
> and Mark's question still stands.
>

Yes. Actually, we use this timestamp for our internal module which is
not yet upstreamed.
The module is necessary to record the logs with the system-wide
timestamp from bootloader to guest OS.


-- 
Best Regards,
Chanho Park