On Wed, Sep 03, 2025 at 07:33:58PM +0000, Hrishabh Rajput wrote:

Gunyah is a Type-I hypervisor which was introduced in the patch series
[1]. It is an open source hypervisor. The source repo is available at
[2].

The Gunyah Hypervisor doesn't allow its Virtual Machines to directly
access the MMIO watchdog. It either provides the fully emulated MMIO
based watchdog interface or the SMC-based watchdog interface depending
on the hypervisor configuration.

EFI provides a standard watchdog interface. Why can't you use that?

The SMC-based watchdog follows ARM's SMC Calling Convention (SMCCC)
version 1.1 and uses Vendor Specific Hypervisor Service Calls space.

Is a watchdog really a hypervisor service? Couldn't a non-virtualized 
OS want to call a watchdog (in secure mode) as well? But I don't know 
how the SMCCC call space is divided up...

This patch series adds support for the SMC-based watchdog interface
provided by the Gunyah Hypervisor. The driver supports start/stop
operations, timeout and pretimeout configuration, pretimeout interrupt
handling and system restart via watchdog.

Shouldn't system restart be handled by PSCI?

Why can't you probe by trying to see if watchdog smc call succeeds to 
see if there is a watchdog? Then you don't need DT for it.
 
Rob