linux-kernel - [PATCH] x86/apic: Use byte array apic_version[], not int array. Saves up to 96k

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Date:   Fri,  9 Sep 2016 10:32:04 +0200
From:   Denys Vlasenko <dvlasenk@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Denys Vlasenko <dvlasenk@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] x86/apic: Use byte array apic_version[], not int array. Saves up to 96k

This array is [MAX_LOCAL_APIC], and MAX_LOCAL_APIC can easily be up to 32k.

This patch changes apic_version[] array elements from int to u8 -
APIC version values as of year 2016 are no larger than 0x1f on all known CPUs.
Version field in the APIC register is 8 bit wide - not likely
to overflow byte range in foreseeable future.

The "ver" argument of generic_processor_info(id,ver), which goes into apic_version[id],
is also changed from int to u8: make it obvious that assignment can't overflow.

generic_processor_info() has four callsites, none of them can put an out-of-range value
into this argument.

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
CC: Ingo Molnar <mingo@...nel.org>
CC: Andy Lutomirski <luto@...capital.net>
CC: "H. Peter Anvin" <hpa@...or.com>
CC: Borislav Petkov <bp@...en8.de>
CC: Brian Gerst <brgerst@...il.com>
CC: x86@...nel.org
CC: linux-kernel@...r.kernel.org
---
 arch/x86/include/asm/mpspec.h | 4 ++--
 arch/x86/kernel/acpi/boot.c   | 2 +-
 arch/x86/kernel/apic/apic.c   | 4 ++--
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/mpspec.h b/arch/x86/include/asm/mpspec.h
index b07233b..a0bc349 100644
--- a/arch/x86/include/asm/mpspec.h
+++ b/arch/x86/include/asm/mpspec.h
@@ -6,7 +6,7 @@
 #include <asm/x86_init.h>
 #include <asm/apicdef.h>
 
-extern int apic_version[];
+extern u8 apic_version[];
 extern int pic_mode;
 
 #ifdef CONFIG_X86_32
@@ -85,7 +85,7 @@ static inline void early_reserve_e820_mpc_new(void) { }
 #define default_get_smp_config x86_init_uint_noop
 #endif
 
-int generic_processor_info(int apicid, int version);
+int generic_processor_info(int apicid, u8 version);
 
 #define PHYSID_ARRAY_SIZE	BITS_TO_LONGS(MAX_LOCAL_APIC)
 
diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index 90d84c3..fde236f 100644
--- a/arch/x86/kernel/acpi/boot.c
+++ b/arch/x86/kernel/acpi/boot.c
@@ -168,7 +168,7 @@ static int __init acpi_parse_madt(struct acpi_table_header *table)
  */
 static int acpi_register_lapic(int id, u32 acpiid, u8 enabled)
 {
-	unsigned int ver = 0;
+	u8 ver = 0;
 	int cpu;
 
 	if (id >= MAX_LOCAL_APIC) {
diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 50c95af..d5cc7c6 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1837,7 +1837,7 @@ void __init register_lapic_address(unsigned long address)
 	}
 }
 
-int apic_version[MAX_LOCAL_APIC];
+u8 apic_version[MAX_LOCAL_APIC];
 
 /*
  * Local APIC interrupts
@@ -2027,7 +2027,7 @@ void disconnect_bsp_APIC(int virt_wire_setup)
 	apic_write(APIC_LVT1, value);
 }
 
-int generic_processor_info(int apicid, int version)
+int generic_processor_info(int apicid, u8 version)
 {
 	int cpu, max = nr_cpu_ids;
 	bool boot_cpu_detected = physid_isset(boot_cpu_physical_apicid,
-- 
2.9.2