[PATCH RFC] x86: Reduce MAX_LOCAL_APIC and MAX_IO_APICS

[Denys Vlasenko <dvlasenk@redhat.com>]

Before this change MAX_LOCAL_APIC had the fixed value of 32*1024.
Such a big value causes several data arrays to be quite oversized:

phys_cpu_present_map is 4 kbytes (one bit per apic id),
__apicid_to_node[] is 64 kbytes,
apic_version[] is 128 kbytes.

On "usual" systems, APIC ids simply go from zero
to maximum logical CPU number, mirroring CPU ids.

On broken and unusual multi-socket systems
APIC ids can be non-contiguous.

This patch changes MAX_LOCAL_APIC definition as follows:

 = It is guaranteed to be at least 16.
 = If NR_CPUS > 16, then it's equal to NR_CPUS.
 = A new CONFIG_MAX_LAPIC_ID can be used to increase it
   (but not decrease).

MAX_IO_APICS was 128. This is a bit large too, making,
for example, ioapics[] array 9216 bytes big.

After this patch, MAX_IO_APICS is at least 8, at most 128.
If NR_CPUS is in this range, then MAX_IO_APICS = NR_CPUS.

apic_version[] array is changed from int to u8 -
APIC version values as of year 2015 are no larger than 0x1f
on all known CPUs.

A bit of code added to ensure that the statement
	apic_version[apicid] = version;
in generic_processor_info() is safe wrt bad values in both
'apicid' and 'version' variables.

This change reduces NR_CPUS=64 kernel's data size by 204661 bytes:

    text     data      bss       dec     hex filename
91353669 13825744 19021824 124201237 7672915 vmlinux.before
91353680 13760336 18882560 123996576 76409a0 vmlinux

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: Ingo Molnar <mingo@kernel.org>
CC: Jiang Liu <jiang.liu@linux.intel.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Len Brown <len.brown@intel.com>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
---
 arch/x86/Kconfig               | 11 +++++++++++
 arch/x86/include/asm/apicdef.h | 23 +++++++++++++++++------
 arch/x86/include/asm/mpspec.h  |  2 +-
 arch/x86/kernel/apic/apic.c    | 19 ++++++++++++++++++-
 4 files changed, 47 insertions(+), 8 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 328c835..9e7c4c1 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -872,6 +872,17 @@ config NR_CPUS
 	  This is purely to save memory - each supported CPU adds
 	  approximately eight kilobytes to the kernel image.
 
+config MAX_LAPIC_ID
+	int "Maximum APIC ID"
+	range 8 32768
+	default "8"
+	---help---
+	  Use this option to set maximum allowed Local APIC ID higher than
+	  maximum number of CPUs. This may be necessary for machines
+	  with large number of processor sockets and non-contiguous
+	  LAPIC numbering.
+	  This setting will be automatically rounded up, if necessary.
+
 config SCHED_SMT
 	bool "SMT (Hyperthreading) scheduler support"
 	depends on SMP
diff --git a/arch/x86/include/asm/apicdef.h b/arch/x86/include/asm/apicdef.h
index c46bb99..64e2476 100644
--- a/arch/x86/include/asm/apicdef.h
+++ b/arch/x86/include/asm/apicdef.h
@@ -147,15 +147,26 @@
 #define XAPIC_ENABLE	(1UL << 11)
 #define X2APIC_ENABLE	(1UL << 10)
 
-#ifdef CONFIG_X86_32
-# define MAX_IO_APICS 64
-# define MAX_LOCAL_APIC 256
-#else
-# define MAX_IO_APICS 128
-# define MAX_LOCAL_APIC 32768
+/*
+ * Allow non-contiguous APIC IDs for small machines:
+ * APIC ids 0..15 are valid in any config.
+ * Typical SMP machines have contiguous APIC IDs: 0..NR_CPUS-1.
+ * CONFIG_MAX_LAPIC_ID can override.
+ */
+#define MAX_LOCAL_APIC (NR_CPUS < 16 ? 16 : NR_CPUS)
+#if MAX_LOCAL_APIC < CONFIG_MAX_LAPIC_ID
+# undef  MAX_LOCAL_APIC
+# define MAX_LOCAL_APIC CONFIG_MAX_LAPIC_ID
 #endif
 
 /*
+ * Minimum is 8.
+ * For largish NR_CPUS, we expect to have no more IOAPICs than CPUs.
+ * No matter how large NR_CPUS is, max is 128.
+ */
+#define MAX_IO_APICS (NR_CPUS < 8 ? 8 : NR_CPUS < 128 ? NR_CPUS : 128)
+
+/*
  * All x86-64 systems are xAPIC compatible.
  * In the following, "apicid" is a physical APIC ID.
  */
diff --git a/arch/x86/include/asm/mpspec.h b/arch/x86/include/asm/mpspec.h
index b07233b..8d0c2e6 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
diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 24e94ce..f49a956 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1798,7 +1798,7 @@ void __init register_lapic_address(unsigned long address)
 	}
 }
 
-int apic_version[MAX_LOCAL_APIC];
+u8 apic_version[MAX_LOCAL_APIC];
 
 /*
  * Local APIC interrupts
@@ -2054,6 +2054,23 @@ int generic_processor_info(int apicid, int version)
 		return -EINVAL;
 	}
 
+	if ((unsigned)apicid >= ARRAY_SIZE(apic_version)) {
+		int thiscpu = max + disabled_cpus;
+		pr_warning("APIC: APIC id 0x%x is too large."
+			   " Processor %d ignored.\n",
+			   apicid, thiscpu);
+		disabled_cpus++;
+		return -EINVAL;
+	}
+	if ((unsigned)version > 255) {
+		int thiscpu = max + disabled_cpus;
+		pr_warning("APIC: APIC version 0x%x is too large."
+			   " Processor %d ignored.\n",
+			   version, thiscpu);
+		disabled_cpus++;
+		return -EINVAL;
+	}
+
 	num_processors++;
 	if (apicid == boot_cpu_physical_apicid) {
 		/*
-- 
1.8.1.4