From: Arnd Bergmann <arnd@arndb.de> Sent: Monday, March 16, 2020 1:30 AM

On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley [off-list ref] wrote:

quoted

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 <redacted>
---
 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.

quoted

+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
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