On Sat, Apr 23, 2022 at 10:36:56AM +0100, Marc Zyngier wrote: > On Sat, 23 Apr 2022 10:02:24 +0100, > Sebastian Ene <sebastianene@xxxxxxxxxx> wrote: > > > > On Sat, Apr 23, 2022 at 08:51:16AM +0200, Greg Kroah-Hartman wrote: > > > On Fri, Apr 22, 2022 at 02:19:48PM +0000, Sebastian Ene wrote: > > > > This adds a mechanism to detect stalls on the guest vCPUS by creating a > > > > per CPU hrtimer which periodically 'pets' the host backend driver. > > > > > > > > This device driver acts as a soft lockup detector by relying on the host > > > > backend driver to measure the elapesed time between subsequent 'pet' events. > > > > If the elapsed time doesn't match an expected value, the backend driver > > > > decides that the guest vCPU is locked and resets the guest. The host > > > > backend driver takes into account the time that the guest is not > > > > running. The communication with the backend driver is done through MMIO > > > > and the register layout of the virtual watchdog is described as part of > > > > the backend driver changes. > > > > > > > > The host backend driver is implemented as part of: > > > > https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3548817 > > > > > > > > Changelog v2: > > > > - move the driver to misc as this does not cope with watchdog core > > > > subsystem > > > > Hello Greg, > > > > > > > > Wait, why does it not cope with it? That's not documented anywhere in > > > your patch that adds the driver. In fact, most of the text here needs > > > to be in the changelog for the driver submission, not thrown away in the > > > 00/XX email that will never end up in the kernel tree. > > > > > > thanks, > > > > > > greg k-h > > > > From the previous feedback that I received on this patch it seems that > > watchdog core is not intended to be used for this type of driver. This > > watchdog device tracks the elapsed time on a per-cpu basis, > > since KVM schedules vCPUs independently. Watchdog core is not intended > > to detect CPU stalls and the drivers don't have a notion of CPU. Hello Marc, > > I must say that I don't really get the objection against the watchdog > approach. OK, there is no userspace aspect to this. But we already > use watchdogs for more than just userspace (reboot is one of the major > use cases). > > There already are per-CPU watchdog in the tree: see how the > fsl-ls208xa platform has one SP805 per CPU (8 of them in total). As > far as I can tell, there was no objection to this. So what is special > about this one? I think the difference is in the fact that this driver expects hrtimers which are CPU binded to execute the periodic watchdog 'pet'. We would require a strong thread affinity setting if we rely on userspace to do this 'pet' operation. Thanks, Sebastian > > Thanks, > > M. > > -- > Without deviation from the norm, progress is not possible.
