Message-ID: <20220706152101.GA3003@willie-the-truck>
Date:   Wed, 6 Jul 2022 16:21:01 +0100
From:   Will Deacon <will@...nel.org>
To:     Sebastian Ene <sebastianene@...gle.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Dragan Cvetic <dragan.cvetic@...inx.com>,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        maz@...nel.org, vdonnefort@...gle.com,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH v9 2/2] misc: Add a mechanism to detect stalls on guest
 vCPUs

Hi Sebastian,

Thanks for working on this and apologies it's taken so long for me to
review your changes.

On Fri, Jul 01, 2022 at 02:40:14PM +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 is
> an emulated MMIO device which is part of the KVM virtual machine
> monitor and this part accounts for lost time by looking at the
> /proc/{}/task/{}/stat entries.
> 
> Signed-off-by: Sebastian Ene <sebastianene@...gle.com>
> ---
>  drivers/misc/Kconfig               |  13 ++
>  drivers/misc/Makefile              |   1 +
>  drivers/misc/vcpu_stall_detector.c | 212 +++++++++++++++++++++++++++++
>  3 files changed, 226 insertions(+)
>  create mode 100644 drivers/misc/vcpu_stall_detector.c
> 
> diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig
> index 41d2bb0ae23a..83afb41a85cf 100644
> --- a/drivers/misc/Kconfig
> +++ b/drivers/misc/Kconfig
> @@ -483,6 +483,19 @@ config OPEN_DICE
>  
>  	  If unsure, say N.
>  
> +config VCPU_STALL_DETECTOR
> +	tristate "VCPU stall detector"

I'd probably stick "Guest" in here somewhere, as this driver won't be
useful on the host. So something like "Guest vCPU stall detector".

> +	select LOCKUP_DETECTOR
> +	depends on OF
> +	help
> +	  Detect CPU locks on a kvm 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 vcpu_stall_detector.

I suggest rewording this along the lines of:

  When this driver is bound inside a KVM guest, it will periodically
  "pet" an MMIO stall detector device from each vCPU and allow the host
  to detect vCPU stalls.

  To compile this driver as a module, choose M here: the module will
  be called vcpu_stall_detector.

  If you do not intend to run this kernel as a guest, say N.

but up to you.

>  source "drivers/misc/c2port/Kconfig"
>  source "drivers/misc/eeprom/Kconfig"
>  source "drivers/misc/cb710/Kconfig"
> diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
> index 70e800e9127f..2be8542616dd 100644
> --- a/drivers/misc/Makefile
> +++ b/drivers/misc/Makefile
> @@ -60,3 +60,4 @@ obj-$(CONFIG_XILINX_SDFEC)	+= xilinx_sdfec.o
>  obj-$(CONFIG_HISI_HIKEY_USB)	+= hisi_hikey_usb.o
>  obj-$(CONFIG_HI6421V600_IRQ)	+= hi6421v600-irq.o
>  obj-$(CONFIG_OPEN_DICE)		+= open-dice.o
> +obj-$(CONFIG_VCPU_STALL_DETECTOR)	+= vcpu_stall_detector.o
> \ No newline at end of file
> diff --git a/drivers/misc/vcpu_stall_detector.c b/drivers/misc/vcpu_stall_detector.c
> new file mode 100644
> index 000000000000..039ac54564c1
> --- /dev/null
> +++ b/drivers/misc/vcpu_stall_detector.c
> @@ -0,0 +1,212 @@
> +// SPDX-License-Identifier: GPL-2.0

This should probably be "GPL-2.0-only" for the avoidance of doubt.

> +//
> +// VCPU stall detector.
> +//  Copyright (C) Google, 2022
> +
> +#include <linux/cpu.h>
> +#include <linux/init.h>
> +#include <linux/io.h>
> +#include <linux/kernel.h>
> +
> +#include <linux/device.h>
> +#include <linux/interrupt.h>
> +#include <linux/module.h>
> +#include <linux/nmi.h>
> +#include <linux/of.h>
> +#include <linux/of_device.h>
> +#include <linux/param.h>
> +#include <linux/percpu.h>
> +#include <linux/platform_device.h>
> +#include <linux/slab.h>
> +
> +#define REG_STATUS		(0x00)
> +#define REG_LOAD_CNT		(0x04)
> +#define REG_CURRENT_CNT		(0x08)
> +#define REG_CLOCK_FREQ_HZ	(0x0C)
> +#define REG_LEN			(0x10)
> +
> +#define DEFAULT_CLOCK_HZ	(10)
> +#define DEFAULT_TIMEOT_SEC	(8)

TIMEOUT instead of TIMEOT?

Generally, I'd probably also prefix these definitions with VCPU_STALL_
or something to avoid the potential for conflicting with the headers.

> +struct vm_stall_detect_s {
> +	void __iomem *membase;
> +	u32 clock_freq;
> +	u32 expiration_sec;
> +	u32 ping_timeout_ms;
> +	struct hrtimer per_cpu_hrtimer;
> +	struct platform_device *dev;
> +};
> +
> +#define vcpu_stall_detect_reg_write(stall_detect, reg, value)	\
> +	iowrite32((value), (stall_detect)->membase + (reg))

You can use writel_relaxed() here as you don't require ordering guarantees
against DMA.

> +#define vcpu_stall_detect_reg_read(stall_detect, reg)		\
> +	io32read((stall_detect)->membase + (reg))

io32read() doesn't exist, but you also don't use this macro so just remove
it :)

> +
> +static struct vm_stall_detect_s __percpu *vm_stall_detect;
> +
> +static enum hrtimer_restart
> +vcpu_stall_detect_timer_fn(struct hrtimer *hrtimer)
> +{
> +	struct vm_stall_detect_s *cpu_stall_detect;
> +	u32 ticks;
> +
> +	cpu_stall_detect = container_of(hrtimer, struct vm_stall_detect_s,
> +					per_cpu_hrtimer);
> +	ticks = cpu_stall_detect->clock_freq * cpu_stall_detect->expiration_sec;
> +	vcpu_stall_detect_reg_write(cpu_stall_detect, REG_LOAD_CNT, ticks);
> +	hrtimer_forward_now(hrtimer,
> +			    ms_to_ktime(cpu_stall_detect->ping_timeout_ms));
> +
> +	return HRTIMER_RESTART;
> +}
> +
> +static void vcpu_stall_detect_start(void *arg)
> +{
> +	u32 ticks;
> +	struct vm_stall_detect_s *cpu_stall_detect = arg;
> +	struct hrtimer *hrtimer = &cpu_stall_detect->per_cpu_hrtimer;
> +
> +	vcpu_stall_detect_reg_write(cpu_stall_detect, REG_CLOCK_FREQ_HZ,
> +				    cpu_stall_detect->clock_freq);
> +
> +	/* Compute the number of ticks required for the stall detector counter
> +	 * register based on the internal clock frequency and the timeout
> +	 * value given from the device tree.
> +	 */
> +	ticks = cpu_stall_detect->clock_freq *
> +		cpu_stall_detect->expiration_sec;
> +	vcpu_stall_detect_reg_write(cpu_stall_detect, REG_LOAD_CNT, ticks);
> +
> +	/* Enable the internal clock and start the stall detector */
> +	vcpu_stall_detect_reg_write(cpu_stall_detect, REG_STATUS, 1);
> +
> +	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> +	hrtimer->function = vcpu_stall_detect_timer_fn;
> +	hrtimer_start(hrtimer, ms_to_ktime(cpu_stall_detect->ping_timeout_ms),
> +		      HRTIMER_MODE_REL_PINNED);
> +}
> +
> +static void vcpu_stall_detect_stop(void *arg)
> +{
> +	struct vm_stall_detect_s *cpu_stall_detect = arg;
> +	struct hrtimer *hrtimer = &cpu_stall_detect->per_cpu_hrtimer;
> +
> +	hrtimer_cancel(hrtimer);
> +
> +	/* Disable the stall detector */
> +	vcpu_stall_detect_reg_write(cpu_stall_detect, REG_STATUS, 0);
> +}
> +
> +static int start_stall_detector_on_cpu(unsigned int cpu)
> +{
> +	vcpu_stall_detect_start(this_cpu_ptr(vm_stall_detect));
> +	return 0;
> +}
> +
> +static int stop_stall_detector_on_cpu(unsigned int cpu)
> +{
> +	vcpu_stall_detect_stop(this_cpu_ptr(vm_stall_detect));
> +	return 0;
> +}
> +
> +static int vcpu_stall_detect_probe(struct platform_device *dev)
> +{
> +	int cpu, ret, err;
> +	void __iomem *membase;
> +	struct resource *r;
> +	u32 stall_detect_clock, stall_detect_timeout_sec = 0;
> +
> +	r = platform_get_resource(dev, IORESOURCE_MEM, 0);
> +	if (r == NULL)
> +		return -ENODEV;
> +
> +	vm_stall_detect = alloc_percpu(typeof(struct vm_stall_detect_s));
> +	if (!vm_stall_detect)
> +		return -ENOMEM;
> +
> +	membase = ioremap(r->start, resource_size(r));
> +	if (!membase) {
> +		ret = -ENOMEM;
> +		goto err_withmem;
> +	}

Can you use devm_platform_get_and_ioremap_resource() to avoid calling
both platform_get_resource() and ioremap() explicitly? I think the devm_
handling there also means you don't need to worry about the iounmap(),
which seems to be missing here anyway.

> +
> +	if (of_property_read_u32(dev->dev.of_node, "clock-frequency",
> +				 &stall_detect_clock))
> +		stall_detect_clock = DEFAULT_CLOCK_HZ;
> +
> +	if (of_property_read_u32(dev->dev.of_node, "timeout-sec",
> +				 &stall_detect_timeout_sec))
> +		stall_detect_timeout_sec = DEFAULT_TIMEOT_SEC;
> +
> +	for_each_cpu_and(cpu, cpu_online_mask, &watchdog_cpumask) {

What prevents CPUs from coming up and down during this loop?

I also don't think we should be using 'watchdog_cpumask' here -- that's
part of the watchdog core; instead we should either take the mask in the
DT description or rely on the device exposing an interface for every CPU
in the system.

> +		struct vm_stall_detect_s *cpu_stall_detect;
> +
> +		cpu_stall_detect = per_cpu_ptr(vm_stall_detect, cpu);
> +		cpu_stall_detect->membase = membase + cpu * REG_LEN;
> +		cpu_stall_detect->clock_freq = stall_detect_clock;
> +		cpu_stall_detect->expiration_sec = stall_detect_timeout_sec;
> +
> +		/* Pet the stall detector at half of its expiration timeout
> +		 * to prevent spurios resets.

typo: spurious

> +		 */
> +		cpu_stall_detect->ping_timeout_ms = stall_detect_timeout_sec *
> +			MSEC_PER_SEC / 2;

All of these fields are the same for each core apart from the membase,
which a CPU can compute locally using its smp_processor_id() so I don't
think you need to include these in the per-cpu structure. It would be
simpler if you tracked these invariants in a separate structure (either
a global static structure or indirected by a per-cpu pointer).

Then the only thing left for the per-cpu structure would be the hrtimer.

> +		smp_call_function_single(cpu, vcpu_stall_detect_start,
> +					 cpu_stall_detect, true);
> +	}
> +
> +	err = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
> +					"virt/vcpu_stall_detector:online",
> +					start_stall_detector_on_cpu,
> +					stop_stall_detector_on_cpu);

If you drop the "_nocalls" variant then I think you can avoid the explicit
previous call to smp_call_function_single() entirely and just rely on the
notifiers to do the right thing. That also solves the hotplug race I
mentioned.

> +	if (err < 0) {
> +		dev_err(&dev->dev, "failed to install cpu hotplug");
> +		ret = err;
> +		goto err_withmem;
> +	}
> +
> +	return 0;
> +
> +err_withmem:
> +	free_percpu(vm_stall_detect);

Looks like there's a devm_alloc_percpu() which might make this a bit
simpler.

> +	return ret;
> +}
> +
> +static int vcpu_stall_detect_remove(struct platform_device *dev)
> +{
> +	int cpu;
> +
> +	for_each_cpu_and(cpu, cpu_online_mask, &watchdog_cpumask) {
> +		struct vm_stall_detect_s *cpu_stall_detect;
> +
> +		cpu_stall_detect = per_cpu_ptr(vm_stall_detect, cpu);
> +		smp_call_function_single(cpu, vcpu_stall_detect_stop,
> +					 cpu_stall_detect, true);
> +	}

I don't think you need to cross-call here, and it looks racy again with
hotplug; you should be able to do something like:

  1. Unregister the hotplug notifier (looks like you're missing this?)
  2. Call hrtimer_cancel() for each percpu timer
  3. Disable the watchdog for each CPU

> +	free_percpu(vm_stall_detect);
> +	vm_stall_detect = NULL;
> +	return 0;
> +}
> +
> +static const struct of_device_id vcpu_stall_detect_of_match[] = {
> +	{ .compatible = "qemu,vcpu-stall-detector", },

I'm not sure why we're mentioning qemu here; something as boring as "virt"
(like you use for the cpuhp notifier) or "dummy" might be better, but given
that Rob is happy with the binding then I won't complain further. We can
always add extra strings later if we need to.

> +	{}
> +};
> +
> +MODULE_DEVICE_TABLE(of, vcpu_stall_detect_of_match);
> +
> +static struct platform_driver vcpu_stall_detect_driver = {
> +	.probe  = vcpu_stall_detect_probe,
> +	.remove = vcpu_stall_detect_remove,
> +	.driver = {
> +		.name           = KBUILD_MODNAME,
> +		.of_match_table = vcpu_stall_detect_of_match,
> +	},
> +};
> +
> +module_platform_driver(vcpu_stall_detect_driver);
> +
> +MODULE_LICENSE("GPL");

This needs to be "GPL v2".

Will

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