linux-kernel - [PATCH] x86/smp: Enforce limit of 512 cpus with MAXSMP and no CPUMASK

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Message-Id: <20191012070054.28657-1-swood@redhat.com>
Date:   Sat, 12 Oct 2019 02:00:54 -0500
From:   Scott Wood <swood@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     Mike Travis <mike.travis@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Scott Wood <swood@...hat.com>
Subject: [PATCH] x86/smp: 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@...hat.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