[PATCH] x86/msr: Use num_possible_cpus() for chrdev minor count

[PATCH] x86/msr: Use num_possible_cpus() for chrdev minor count

[Roman 'Hedin' Storozhenko]

The MSR driver currently register their character device
regions using NR_CPUS as the minor count:

  __register_chrdev(..., 0, NR_CPUS, ...)

This dates back to commit 0b962d473af3 ("x86, msr/cpuid: Register
enough minors for the MSR and CPUID drivers"), which intentionally
used NR_CPUS as a static upper bound on the maximum CPU index.

However, NR_CPUS is a compile-time upper bound and may significantly
exceed the actual number of CPUs supported by the running system
(i.e. cpu_possible_mask). As a result, the drivers may reserve a much
larger minor range than can ever be used at runtime.

Replace NR_CPUS with num_possible_cpus() so that the registered minor
range reflects the actual system topology instead of a static maximum.

The driver already creates and destroy devices dynamically based on
CPU hotplug state, so this change only affects the size of the
internally reserved minor range and does not alter runtime behavior
or userspace-visible interfaces.

Signed-off-by: Roman 'Hedin' Storozhenko <romeusmeister@gmail.com>
---
This series replaces NR_CPUS with num_possible_cpus() when registering
character device regions for the MSR and CPUID drivers.

Historically, commit 0b962d473af3 ("x86, msr/cpuid: Register enough minors
for the MSR and CPUID drivers") switched to using NR_CPUS as a static upper
bound on the maximum CPU index. While correct, this may result in reserving
a significantly larger minor range than is actually usable on a given system.

This series updates the drivers to use num_possible_cpus(), aligning the
registered minor range with the system’s CPU topology.

The change does not affect runtime behavior: devices are still created and
destroyed dynamically based on CPU hotplug state.

Validation
----------

The change was validated in a virtme-ng environment with:

  CONFIG_NR_CPUS=64
  possible CPUs: 0-7 (8 CPUs)

1) Internal state (eBPF)

Using bpftrace on __register_chrdev_region():

  Before:
    count = 64

  After:
    count = 8

This confirms that the registered minor range now reflects
num_possible_cpus() instead of NR_CPUS.

2) Userspace-visible behavior

No change observed:

  /dev/cpu/*/msr count: unchanged (8)
  /sys/class/msr entries: unchanged (8)

3) Functional correctness

rdmsr access continues to work as expected.

4) CPU hotplug

Tracing msr_device_create shows identical behavior before and after,
confirming that device lifecycle remains driven by CPU hotplug events.

----------
Overall, this change reduces over-allocation of minor numbers while
preserving existing behavior and interfaces.
---
 arch/x86/kernel/msr.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index 4469c784eaa0..7f93f59d3cd9 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -263,7 +263,7 @@ static int __init msr_init(void)
 {
 	int err;
 
-	if (__register_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr", &msr_fops)) {
+	if (__register_chrdev(MSR_MAJOR, 0, num_possible_cpus(), "cpu/msr", &msr_fops)) {
 		pr_err("unable to get major %d for msr\n", MSR_MAJOR);
 		return -EBUSY;
 	}
@@ -281,7 +281,7 @@ static int __init msr_init(void)
 out_class:
 	class_unregister(&msr_class);
 out_chrdev:
-	__unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
+	__unregister_chrdev(MSR_MAJOR, 0, num_possible_cpus(), "cpu/msr");
 	return err;
 }
 module_init(msr_init);
@@ -290,7 +290,7 @@ static void __exit msr_exit(void)
 {
 	cpuhp_remove_state(cpuhp_msr_state);
 	class_unregister(&msr_class);
-	__unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
+	__unregister_chrdev(MSR_MAJOR, 0, num_possible_cpus(), "cpu/msr");
 }
 module_exit(msr_exit)
 

---
base-commit: 25f9333c7244d9c5a0243bcf045903ba19635d35
change-id: 20260401-msr_cpu-d88ade162ca0

Best regards,
-- 
Roman 'Hedin' Storozhenko <romeusmeister@gmail.com>

RE: [PATCH] x86/msr: Use num_possible_cpus() for chrdev minor count

[Michael Kelley]

From: Roman 'Hedin' Storozhenko <romeusmeister@gmail.com> Sent: Wednesday, April 1, 2026 11:56 AM
> 
> The MSR driver currently register their character device
> regions using NR_CPUS as the minor count:
> 
>   __register_chrdev(..., 0, NR_CPUS, ...)
> 
> This dates back to commit 0b962d473af3 ("x86, msr/cpuid: Register
> enough minors for the MSR and CPUID drivers"), which intentionally
> used NR_CPUS as a static upper bound on the maximum CPU index.
> 
> However, NR_CPUS is a compile-time upper bound and may significantly
> exceed the actual number of CPUs supported by the running system
> (i.e. cpu_possible_mask). As a result, the drivers may reserve a much
> larger minor range than can ever be used at runtime.
> 
> Replace NR_CPUS with num_possible_cpus() so that the registered minor
> range reflects the actual system topology instead of a static maximum.

Using num_possible_cpus() introduces the potential for a different
problem. There's no guarantee that the cpu_possible_mask is dense,
so num_possible_cpus() might be smaller than the largest possible
Linux cpu number. Since msr_device_create() uses the Linux cpu number
as the minor device number, that minor number could exceed the size
of the registered minor range.

There's no problem seen today because I believe the possible_cpus
bitmap is currently always dense, but there's the potential in the future
that it is not. See email thread [1].

Instead of num_possible_cpus(), use nr_cpu_ids.

Michael

[1] https://lore.kernel.org/lkml/SN6PR02MB4157210CC36B2593F8572E5ED4692@SN6PR02MB4157.namprd02.prod.outlook.com/

> 
> The driver already creates and destroy devices dynamically based on
> CPU hotplug state, so this change only affects the size of the
> internally reserved minor range and does not alter runtime behavior
> or userspace-visible interfaces.
> 
> Signed-off-by: Roman 'Hedin' Storozhenko <romeusmeister@gmail.com>
> ---
> This series replaces NR_CPUS with num_possible_cpus() when registering
> character device regions for the MSR and CPUID drivers.
> 
> Historically, commit 0b962d473af3 ("x86, msr/cpuid: Register enough minors
> for the MSR and CPUID drivers") switched to using NR_CPUS as a static upper
> bound on the maximum CPU index. While correct, this may result in reserving
> a significantly larger minor range than is actually usable on a given system.
> 
> This series updates the drivers to use num_possible_cpus(), aligning the
> registered minor range with the system's CPU topology.
> 
> The change does not affect runtime behavior: devices are still created and
> destroyed dynamically based on CPU hotplug state.
> 
> Validation
> ----------
> 
> The change was validated in a virtme-ng environment with:
> 
>   CONFIG_NR_CPUS=64
>   possible CPUs: 0-7 (8 CPUs)
> 
> 1) Internal state (eBPF)
> 
> Using bpftrace on __register_chrdev_region():
> 
>   Before:
>     count = 64
> 
>   After:
>     count = 8
> 
> This confirms that the registered minor range now reflects
> num_possible_cpus() instead of NR_CPUS.
> 
> 2) Userspace-visible behavior
> 
> No change observed:
> 
>   /dev/cpu/*/msr count: unchanged (8)
>   /sys/class/msr entries: unchanged (8)
> 
> 3) Functional correctness
> 
> rdmsr access continues to work as expected.
> 
> 4) CPU hotplug
> 
> Tracing msr_device_create shows identical behavior before and after,
> confirming that device lifecycle remains driven by CPU hotplug events.
> 
> ----------
> Overall, this change reduces over-allocation of minor numbers while
> preserving existing behavior and interfaces.
> ---
>  arch/x86/kernel/msr.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
> index 4469c784eaa0..7f93f59d3cd9 100644
> --- a/arch/x86/kernel/msr.c
> +++ b/arch/x86/kernel/msr.c
> @@ -263,7 +263,7 @@ static int __init msr_init(void)
>  {
>  	int err;
> 
> -	if (__register_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr", &msr_fops)) {
> +	if (__register_chrdev(MSR_MAJOR, 0, num_possible_cpus(), "cpu/msr",
> &msr_fops)) {
>  		pr_err("unable to get major %d for msr\n", MSR_MAJOR);
>  		return -EBUSY;
>  	}
> @@ -281,7 +281,7 @@ static int __init msr_init(void)
>  out_class:
>  	class_unregister(&msr_class);
>  out_chrdev:
> -	__unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
> +	__unregister_chrdev(MSR_MAJOR, 0, num_possible_cpus(), "cpu/msr");
>  	return err;
>  }
>  module_init(msr_init);
> @@ -290,7 +290,7 @@ static void __exit msr_exit(void)
>  {
>  	cpuhp_remove_state(cpuhp_msr_state);
>  	class_unregister(&msr_class);
> -	__unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
> +	__unregister_chrdev(MSR_MAJOR, 0, num_possible_cpus(), "cpu/msr");
>  }
>  module_exit(msr_exit)
> 
> 
> ---
> base-commit: 25f9333c7244d9c5a0243bcf045903ba19635d35
> change-id: 20260401-msr_cpu-d88ade162ca0
> 
> Best regards,
> --
> Roman 'Hedin' Storozhenko <romeusmeister@gmail.com>
>