linux-kernel - Re: [PATCH v4 2/2] misc: Add a mechanism to detect stalls on guest vCPUs

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Date:   Sat, 30 Apr 2022 08:18:14 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Sebastian Ene <sebastianene@...gle.com>,
        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 v4 2/2] misc: Add a mechanism to detect stalls on guest
 vCPUs

On Fri, Apr 29, 2022 at 09:51:51AM -0700, Guenter Roeck wrote:
> On 4/29/22 01:48, Greg Kroah-Hartman wrote:
> > On Fri, Apr 29, 2022 at 08:30:33AM +0000, Sebastian Ene wrote:
> > > This driver creates per-cpu hrtimers which are required to do the
> > > periodic 'pet' operation. On a conventional watchdog-core driver, the
> > > userspace is responsible for delivering the 'pet' events by writing to
> > > the particular /dev/watchdogN node. In this case we require a strong
> > > thread affinity to be able to account for lost time on a per vCPU.
> > > 
> > > This part of the driver is the 'frontend' which is reponsible for
> > > delivering the periodic 'pet' events, configuring the virtual peripheral
> > > and listening for cpu hotplug events. The other part of the driver
> > > handles the peripheral emulation and this part accounts for lost time by
> > > looking at the /proc/{}/task/{}/stat entries and is located here:
> > > https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3548817
> > > 
> > > Signed-off-by: Sebastian Ene <sebastianene@...gle.com>
> > > ---
> > >   drivers/misc/Kconfig       |  12 +++
> > >   drivers/misc/Makefile      |   1 +
> > >   drivers/misc/vm-watchdog.c | 206 +++++++++++++++++++++++++++++++++++++
> > >   3 files changed, 219 insertions(+)
> > >   create mode 100644 drivers/misc/vm-watchdog.c
> > > 
> > > diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig
> > > index 2b9572a6d114..26c3a99e269c 100644
> > > --- a/drivers/misc/Kconfig
> > > +++ b/drivers/misc/Kconfig
> > > @@ -493,6 +493,18 @@ config OPEN_DICE
> > >   	  If unsure, say N.
> > > +config VM_WATCHDOG
> > > +	tristate "Virtual Machine Watchdog"
> > > +	select LOCKUP_DETECTOR
> > > +	help
> > > +	  Detect CPU locks on the virtual machine. This driver relies on the
> > > +	  hrtimers which are CPU-binded to do the 'pet' operation. When a vCPU
> > > +	  has to do a 'pet', it exits the guest through MMIO write and the
> > > +	  backend driver takes into account the lost ticks for this particular
> > > +	  CPU.
> > > +	  To compile this driver as a module, choose M here: the
> > > +	  module will be called vm-wdt.
> > 
> > You forgot to name the module properly here based on the Makefile change
> > you made.
> > 
> > And again, as this is called a "watchdog", it seems crazy that it is not
> > in drivers/watchdog/
> > 
> 
> I disagree. It is not a watchdog driver in the traditional sense (it does
> not use, want to use, or need to use the watchdog driver API or ABI).
> Its functionality is similar to the functionality of kernel/watchdog.c,
> which doesn't belong into drivers/watchdog either.

Ah, ok, that makes more sense, the user/kernel api is not the same.
Someone should put that in the changelog next time :)

thanks,

greg k-h