Re: Very Early boot time stamps support

[Abderrahmane Benbachir]

Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> a ÃcritÂ:

> Hi Abderrahmane,
>
> > I'm implementing a feature in ftrace to enable very early function tracing,
> > I'm using tsc when x86_tsc feature is available, but it seems that you did
> > similar work in your patch "[PATCH v9 0/6] Early boot time stamps for x86".
> >
> > I need to record timestamps at the start of start_kernel(), the first
> > function to be traced is set_task_stack_end_magic.
>
> Early boot time stamps are available only after platforms are
> initialized. This happens in:
>
> start_kernel()
>  setup_arch()
>   kvmclock_init() <-- right after this call.
>
> This was a request from Thomas, not to invent a new TSC calibration,
> and instead rely on what is already available.
This seems related to kvm/hypervisor, is there any available TSC calibration
that could be used independently from kvm ?

> So, while not covering the whole boot process, early boot clock is
> still initialized pretty early.
>
> Here a few ideas how to make early boot clock to initialize earlier.
>
> 2. We could initialize TSC earlier only on those CPUs where hardware
> tells us specifically TSC frequency i.e. cpuinfo(15) is available. See
> native_calibrate_tsc() for an example. This is true for all the latest
> Intel CPUs. Unfortunately, this does not work with qemu, as it returns
> some garbage in cpuinfo(15), even when processor supports this feature
> natively.
Is this a bug in Qemu ?