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[34.77.222.49]) by smtp.gmail.com with ESMTPSA id h7-20020adfa4c7000000b0020c5253d907sm11000665wrb.83.2022.05.04.00.29.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 May 2022 00:29:24 -0700 (PDT) Date: Wed, 4 May 2022 07:29:23 +0000 From: Sebastian Ene To: Rob Herring Cc: linux-kernel@vger.kernel.org, Derek Kiernan , Dragan Cvetic , Arnd Bergmann , devicetree@vger.kernel.org, qperret@google.com, will@kernel.org, maz@kernel.org, Greg Kroah-Hartman Subject: Re: [PATCH v4 2/2] misc: Add a mechanism to detect stalls on guest vCPUs Message-ID: References: <20220429083030.3241640-1-sebastianene@google.com> <20220429083030.3241640-3-sebastianene@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: devicetree@vger.kernel.org On Fri, Apr 29, 2022 at 04:03:45PM -0500, Rob Herring wrote: > On Fri, Apr 29, 2022 at 11:38:52AM +0200, Greg Kroah-Hartman wrote: > > On Fri, Apr 29, 2022 at 09:26:26AM +0000, Sebastian Ene wrote: > > > On Fri, Apr 29, 2022 at 10:51:14AM +0200, 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 > > > > > --- > > > > > 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. > > > > > > Hi, > > > > > > > > > > > There's nothing to keep this tied to a virtual machine at all, right? > > > > You are just relying on some iomem address to be updated, so it should > > > > be a "generic_iomem_watchdog" driver as there's nothing specific to vms > > > > at all from what I can tell. > > > > > > > > thanks, > > > > > > > > greg k-h > > > > > > That's right although I might think of using the term "generic lockup detector" > > > instead of watchdog. The only reason why I would keep "virtual machine" > > > word in, is that there is no actual hardware for this. > > > > That doesn't really matter, it's just a memory location in device tree > > that you are needing, odds are some hardware device could use it just > > like this. Hi, > > Such as a shared on-chip memory that both a system control processor and > the main processors can access. Of course, those also typically already > have a comnunication channel. > > But for a VM-hypervisor interface, why isn't one of the existing > communications interfaces being used? One that is discoverable would be > better than using DT. > In a protected VM we don't trust the host to present and control the loaded peripherals. We rely on another entity to generate a trusted device tree for us. I hope this clarifies the need for DT and I think this information should also be added in the changelog. > Rob Thanks, Seb