[PATCH] x86/smp: Enforce limit of 512 cpus with MAXSMP and no CPUMASK_OFFSTACK

[PATCH] x86/smp: Enforce limit of 512 cpus with MAXSMP and no CPUMASK_OFFSTACK

[Scott Wood]

The help text of NR_CPUS says that the maximum number of cpus supported
without CPUMASK_OFFSTACK is 512.  However, NR_CPUS_RANGE_END allows this
limit to be bypassed by MAXSMP even if CPUMASK_OFFSTACK is not set.

This scenario can currently only happen in the RT tree, since it
has "select CPUMASK_OFFSTACK if !PREEMPT_RT_FULL" in MAXSMP.  However,
even if we ignore the RT tree, checking for MAXSMP in addition to
CPUMASK_OFFSTACK is redundant.

Signed-off-by: Scott Wood <swood@redhat.com>
---
 arch/x86/Kconfig | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index d6e1faa28c58..2b526e101dc9 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1000,8 +1000,8 @@ config NR_CPUS_RANGE_END
 config NR_CPUS_RANGE_END
 	int
 	depends on X86_64
-	default 8192 if  SMP && ( MAXSMP ||  CPUMASK_OFFSTACK)
-	default  512 if  SMP && (!MAXSMP && !CPUMASK_OFFSTACK)
+	default 8192 if  SMP && CPUMASK_OFFSTACK
+	default  512 if  SMP && !CPUMASK_OFFSTACK
 	default    1 if !SMP
 
 config NR_CPUS_DEFAULT
-- 
2.20.1

[tip: x86/cpu] x86/Kconfig: Enforce limit of 512 CPUs with MAXSMP and no CPUMASK_OFFSTACK

[tip-bot2 for Scott Wood]

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     1edae1ae62589f28d00da186465a003e2a7f9c6c
Gitweb:        https://git.kernel.org/tip/1edae1ae62589f28d00da186465a003e2a7f9c6c
Author:        Scott Wood <swood@redhat.com>
AuthorDate:    Sat, 12 Oct 2019 02:00:54 -05:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Thu, 17 Oct 2019 18:04:51 +02:00

x86/Kconfig: Enforce limit of 512 CPUs with MAXSMP and no CPUMASK_OFFSTACK

The help text of NR_CPUS says that the maximum number of CPUs supported
without CPUMASK_OFFSTACK is 512. However, NR_CPUS_RANGE_END allows this
limit to be bypassed by MAXSMP even if CPUMASK_OFFSTACK is not set.

This scenario can currently only happen in the RT tree, since it has
"select CPUMASK_OFFSTACK if !PREEMPT_RT_FULL" in MAXSMP. However,
even if we ignore the RT tree, checking for MAXSMP in addition to
CPUMASK_OFFSTACK is redundant.

Signed-off-by: Scott Wood <swood@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mike Travis <mike.travis@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20191012070054.28657-1-swood@redhat.com
---
 arch/x86/Kconfig | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 91c22ee..896f840 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1000,8 +1000,8 @@ config NR_CPUS_RANGE_END
 config NR_CPUS_RANGE_END
 	int
 	depends on X86_64
-	default 8192 if  SMP && ( MAXSMP ||  CPUMASK_OFFSTACK)
-	default  512 if  SMP && (!MAXSMP && !CPUMASK_OFFSTACK)
+	default 8192 if  SMP && CPUMASK_OFFSTACK
+	default  512 if  SMP && !CPUMASK_OFFSTACK
 	default    1 if !SMP
 
 config NR_CPUS_DEFAULT