On Thu, Sep 4, 2025 at 6:31 AM Konrad Dybcio <konrad.dybcio@xxxxxxxxxxxxxxxx> wrote: > > On 9/4/25 2:10 AM, Rob Herring wrote: > > 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 use of UEFI at Qualcomm is not exactly what you would expect.. > > > > >> 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... > > Gunyah traps SMC calls and acts on a subset of them, passing others > to TZ My question was just whether it's the right call space to use. I would think hypervisor calls would be things like "vm start" or "vm stop", not something which in theory could be implemented without a hypervisor in the middle. > >> 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? > > I believe the author is trying to say that the watchdog is not > configurable from Linux at present, and if the platform hangs, there > are some indeterminate default settings in place > > > > > 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. > > There apparently isn't a good way to tell from a running system whether > Gunyah is present, unless you make a smc call (which could in theory be > parsed by something else, say a different hypervisor..), but then this > patch only introduces the watchdog interface, without all the cruft that > would actually let us identify the hypervisor, get its version ID and > perform sanity checks.. IIRC, last time we got just a gunyah node. Now it's that plus a watchdog. What's next? I'm not really a fan of $soc_vendor hypervisor interfaces. I doubt anyone else is either. We have all sorts of standard interfaces already between virtio, vfio, EFI, SCMI, PSCI, etc. Can we please not abuse DT with $soc_vendor hypervisor devices. Rob
