RE: [PATCH v6 07/10] arm64: hyperv: Initialize hypervisor on boot

[Michael Kelley]

From: Arnd Bergmann <arnd@xxxxxxxx> Sent: Monday, March 16, 2020 1:30 AM
> 
> On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley <mikelley@xxxxxxxxxxxxx> wrote:
> >
> > Add ARM64-specific code to initialize the Hyper-V
> > hypervisor when booting as a guest VM. Provide functions
> > and data structures indicating hypervisor status that
> > are needed by VMbus driver.
> >
> > This code is built only when CONFIG_HYPERV is enabled.
> >
> > Signed-off-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> > ---
> >  arch/arm64/hyperv/hv_core.c | 156
> ++++++++++++++++++++++++++++++++++++++++++++
> 
> As you are effectively adding a new clocksource driver here, please move the
> code to drivers/clocksource and send the patch to the respective maintainers
> (added to Cc here), splitting it out from the rest of the patch.
> 
> You should also describe why your platform doesn't just use the normal
> architected timer interface.
> 
> > +TIMER_ACPI_DECLARE(hyperv, ACPI_SIG_GTDT, hyperv_init);
> 
> This looks like it registers a driver for the same device as the normal
> arch timer. Won't that clash?
> 
>      Arnd

There is a Hyper-V clocksource driver in drivers/clocksource/hyperv_timer.c.
It is architecture independent and works for both x86 and ARM64.

The requirement here is really for a place to hang the general Hyper-V
initialization code.   On the x86 side, there's infrastructure already in place
to do hypervisor initialization, but nothing corresponding on the ARM64 side.
The TIMER_ACPI_DECLARE hook is admittedly a temporary approach, and I'm
happy to hear if someone has a better way to handle this.

FWIW, Hyper-V doesn't currently virtualize the ARM arch counter/timer for
guest VMs.  The Hyper-V synthetic counter/timer in the Hyper-V clocksource
driver is used on both ARM64 and x86.  But this Hyper-V init code doesn't actually
touch the GTDT device, so it won't interfere with the ARM arch counter/timer
when a future Hyper-V version does virtualize it.

Michael