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

[Shrikanth Hegde]

Hi Yury,

On 7/7/26 12:19 AM, Yury Norov wrote:
> On Mon, Jul 06, 2026 at 01:54:05PM +0530, Shrikanth Hegde wrote:
> > On 7/3/26 11:57 PM, Yury Norov wrote:
> > > On Fri, Jul 03, 2026 at 02:20:32PM -0400, Yury Norov wrote:
> > > > 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.
> > >
> > > What if I've got my own monitor, and don't need this one? Please add a
> > > way to not compile it, even if CONFIG_PREFERRED_CPU is enabled.
> >
> > You mean make a new config like VIRT_MONITOR depend on CONFIG_PREFERRED_CPU and
> > make it as default n?
> >
> > Can we defer this until such a new monitor is needed?
> > We will in state where CONFIG_PREFERRED_CPU=y & VIRT_MONITOR=n.
>
> I think the VIRT_MONITOR should select PREFERRED_CPU. Whether
> PREFERRED_CPU should be enabled without an in-tree driver using it -
> IDK. Probably yes.

That's good idea too. User has to enable VIRT_MONITOR, that will select
PREFERRED_CPU. If any other driver wants this feature, it will have to do the
same. Assumption is whichever driver selects PREFERRED_CPU, it has to ensure that
design constrains are met.

I did below. I think this is what you meant. Let me know if i got it wrong.

---

diff --git a/drivers/virt/Kconfig b/drivers/virt/Kconfig
index 52eb7e4ba71f..a52233b2502e 100644
--- a/drivers/virt/Kconfig
+++ b/drivers/virt/Kconfig
@@ -47,6 +47,8 @@ source "drivers/virt/nitro_enclaves/Kconfig"
 
 source "drivers/virt/acrn/Kconfig"
 
+source "drivers/virt/steal_monitor/Kconfig"
+
 endif
 
 source "drivers/virt/coco/Kconfig"
diff --git a/drivers/virt/Makefile b/drivers/virt/Makefile
index aff715cea42d..b67fd8968ec3 100644
--- a/drivers/virt/Makefile
+++ b/drivers/virt/Makefile
@@ -9,5 +9,5 @@ obj-y                           += vboxguest/
 
 obj-$(CONFIG_NITRO_ENCLAVES)   += nitro_enclaves/
 obj-$(CONFIG_ACRN_HSM)         += acrn/
-obj-$(CONFIG_PREFERRED_CPU)    += steal_monitor/
+obj-$(CONFIG_STEAL_MONITOR)    += steal_monitor/
 obj-y                          += coco/
diff --git a/drivers/virt/steal_monitor/Kconfig b/drivers/virt/steal_monitor/Kconfig
new file mode 100644
index 000000000000..efb94b3a60e4
--- /dev/null
+++ b/drivers/virt/steal_monitor/Kconfig
@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: GPL-2.0-only
+config STEAL_MONITOR
+       tristate "Dynamic vCPU management based on steal time"
+       depends on PARAVIRT && SMP
+       select PREFERRED_CPU
+       default m
+       help
+         This feature helps to reduce the steal time in paravirtualised
+         environment, there by reducing vCPU preemption. Reducing vCPU
+         preemption provides improved lock holder preemption and reduces
+         cost of vCPU preemption in the host.
+
+         By default preferred CPUs will be same as active CPUs. Depending
+         on the steal time when steal_monitor driver is enabled,
+         preferred CPUs could become subset of active CPUs.
diff --git a/drivers/virt/steal_monitor/Makefile b/drivers/virt/steal_monitor/Makefile
index 7c16f8cf9583..d206e048a9ea 100644
--- a/drivers/virt/steal_monitor/Makefile
+++ b/drivers/virt/steal_monitor/Makefile
@@ -9,6 +9,6 @@
 # It is always compiled as module if CONFIG_PREFERRED_CPU=y
 # One has to enable the module.
 #
-obj-$(subst y,m,$(CONFIG_PREFERRED_CPU)) += steal_monitor.o
+obj-$(CONFIG_STEAL_MONITOR) += steal_monitor.o
 
 steal_monitor-y := sm_core.o defaults.o
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 01b908ff1740..ed02e4431230 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -194,15 +194,4 @@ config SCHED_CLASS_EXT
            https://github.com/sched-ext/scx
 
 config PREFERRED_CPU
-       bool "Dynamic vCPU management based on steal time"
-       depends on PARAVIRT && SMP
-       default y
-       help
-         This feature helps to reduce the steal time in paravirtualised
-         environment, thereby reducing vCPU preemption. Reducing vCPU
-         preemption provides improved lock holder preemption and reduces
-         cost of vCPU preemption.
-
-         By default preferred CPUs will be same as active CPUs. Depending
-         on the steal time when steal_monitor driver is enabled,
-         preferred CPUs could become subset of active CPUs.
+       bool


> Enabling some random driver, and even worse overriding user
> preferences looks questionable, at least.
>
> I'm one of those who build VMs with the localyes config, particularly
> because it decouples me from rootfs headache. So please let me just
> compile-in your monitor, if I want.