linux-kernel - Re: [PATCH v2 0/2] Detect stalls on guest vCPUS

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Date:   Sat, 23 Apr 2022 08:51:16 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sebastian Ene <sebastianene@...gle.com>
Cc:     Rob Herring <robh+dt@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Dragan Cvetic <dragan.cvetic@...inx.com>,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        maz@...nel.org, will@...nel.org, qperret@...gle.com
Subject: Re: [PATCH  v2 0/2] Detect stalls on guest vCPUS

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

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