Re: [PATCH v6 12/23] virt: Introduce steal monitor driver

[Yury Norov <ynorov@nvidia.com>]

On Wed, Jul 01, 2026 at 07:46:43PM +0530, Shrikanth Hegde wrote:
> Introduce a new driver in virt named steal_monitor. This driver
> will compute the steal time and drive the policy decisions of preferred
> CPU state.
> 
> More on it can be found in the Documentation/driver-api/steal-monitor.rst
> Introduce the skeleton code first.
> 
> There is no new kconfig. It depends on CONFIG_PREFERRED_CPU.
> - If CONFIG_PREFERRED_CPU=y, it gets compiled as a module. It is not
>   loaded by default.
> - If CONFIG_PREFERRED_CPU=n, module isn't compiled.
> 
> File layout of the driver is designed with having arch specific
> files in the future.
> 
> - sm_core.c - contains main driver code. This includes the periodic
>   work function and take action on steal time.
> - defaults.c - contains the default implementation defined with __weak
>   symbols.
> - sm_core.h - header file which includes data structure.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>

You've split the driver code into 12 patches. It makes it impossible
to review the driver as a whole. Please make it less granular. 2 or 3
patches for the new driver is more than enough, I think.

> ---
>  drivers/virt/Makefile                |  1 +
>  drivers/virt/steal_monitor/Makefile  | 14 ++++++++++++
>  drivers/virt/steal_monitor/sm_core.c | 33 ++++++++++++++++++++++++++++
>  drivers/virt/steal_monitor/sm_core.h | 11 ++++++++++
>  4 files changed, 59 insertions(+)
>  create mode 100644 drivers/virt/steal_monitor/Makefile
>  create mode 100644 drivers/virt/steal_monitor/sm_core.c
>  create mode 100644 drivers/virt/steal_monitor/sm_core.h
> 
> diff --git a/drivers/virt/Makefile b/drivers/virt/Makefile
> index f29901bd7820..aff715cea42d 100644
> --- a/drivers/virt/Makefile
> +++ b/drivers/virt/Makefile
> @@ -9,4 +9,5 @@ obj-y				+= vboxguest/
>  
>  obj-$(CONFIG_NITRO_ENCLAVES)	+= nitro_enclaves/
>  obj-$(CONFIG_ACRN_HSM)		+= acrn/
> +obj-$(CONFIG_PREFERRED_CPU)	+= steal_monitor/
>  obj-y				+= coco/
> diff --git a/drivers/virt/steal_monitor/Makefile b/drivers/virt/steal_monitor/Makefile
> new file mode 100644
> index 000000000000..24cee55342ce
> --- /dev/null
> +++ b/drivers/virt/steal_monitor/Makefile
> @@ -0,0 +1,14 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +#
> +# Steal time monitor to alter preferred CPU state.
> +#
> +# Arch can implement strong function definitions and override the
> +# default by adding them in arch specific file. It must ensure
> +# that preferred is always subset of active.
> +#
> +# It is always compiled as module if CONFIG_PREFERRED_CPU=y
> +# One has to enable the module.

Why?

> +#
> +obj-$(subst y,m,$(CONFIG_PREFERRED_CPU)) += steal_monitor.o
> +
> +steal_monitor-y := sm_core.o
> diff --git a/drivers/virt/steal_monitor/sm_core.c b/drivers/virt/steal_monitor/sm_core.c
> new file mode 100644
> index 000000000000..e320559c6576
> --- /dev/null
> +++ b/drivers/virt/steal_monitor/sm_core.c
> @@ -0,0 +1,33 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Steal time Monitor.
> + *
> + * Periodically compute steal time. Based on the thresholds either
> + * reduce/increase the preferred CPUs which can be made use
> + * by the workload to avoid vCPU preemption to an extent possible.
> + *
> + * Available as module with CONFIG_PREFERRED_CPU=y
> + *
> + * Copyright (C) 2026 IBM
> + * Author: Shrikanth Hegde <sshegde@linux.ibm.com>
> + */
> +
> +#include "sm_core.h"
> +
> +static int __init steal_monitor_init(void)
> +{
> +	pr_info("steal_monitor is enabled\n");
> +	return 0;
> +}
> +
> +static void __exit steal_monitor_exit(void)
> +{
> +	pr_info("steal_monitor is disabled\n");
> +}
> +
> +module_init(steal_monitor_init);
> +module_exit(steal_monitor_exit);
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("IBM Corporation");
> +MODULE_DESCRIPTION("Virtualization Steal Time Monitor");
> diff --git a/drivers/virt/steal_monitor/sm_core.h b/drivers/virt/steal_monitor/sm_core.h
> new file mode 100644
> index 000000000000..684a258526e1
> --- /dev/null
> +++ b/drivers/virt/steal_monitor/sm_core.h
> @@ -0,0 +1,11 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +#ifndef __VIRT_STEAL_CORE_H
> +#define __VIRT_STEAL_CORE_H
> +
> +#include <linux/types.h>
> +
> +#include <linux/module.h>
> +#include <linux/kernel.h>
> +#include <linux/init.h>
> +
> +#endif /* __VIRT_STEAL_CORE_H */
> -- 
> 2.47.3