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Date:   Sat, 23 Apr 2022 09:02:24 +0000
From:   Sebastian Ene <sebastianene@google.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org,
        Derek Kiernan <derek.kiernan@xilinx.com>,
        Dragan Cvetic <dragan.cvetic@xilinx.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Rob Herring <robh+dt@kernel.org>, devicetree@vger.kernel.org,
        qperret@google.com, will@kernel.org, maz@kernel.org
Subject: Re: [PATCH  v2 0/2] Detect stalls on guest vCPUS
Message-ID: <YmPAoFjfGoBHbPjC@google.com>
References: <20220422141949.3456505-1-sebastianene@google.com>
 <YmOh5I7ZK08LlM6D@kroah.com>
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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.

Thanks,
Sebastian