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x86/segments: Introduce the 'CPUNODE' naming to better document the segment limit CPU/node NR trick

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We have a special segment descriptor entry in the GDT, whose sole purpose is to
encode the CPU and node numbers in its limit (size) field. There are user-space
instructions that allow the reading of the limit field, which gives us a really
fast way to read the CPU and node IDs from the vDSO for example.

But the naming of related functionality does not make this clear, at all:

	VDSO_CPU_SIZE
	VDSO_CPU_MASK
	__CPU_NUMBER_SEG
	GDT_ENTRY_CPU_NUMBER
	vdso_encode_cpu_node
	vdso_read_cpu_node

There's a number of problems:

 - The 'VDSO_CPU_SIZE' doesn't really make it clear that these are number
   of bits, nor does it make it clear which 'CPU' this refers to, i.e.
   that this is about a GDT entry whose limit encodes the CPU and node number.

 - Furthermore, the 'CPU_NUMBER' naming is actively misleading as well,
   because the segment limit encodes not just the CPU number but the
   node ID as well ...

So use a better nomenclature all around: name everything related to this trick
as 'CPUNODE', to make it clear that this is something special, and add
_BITS to make it clear that these are number of bits, and propagate this to
every affected name:

	VDSO_CPU_SIZE         =>  VDSO_CPUNODE_BITS
	VDSO_CPU_MASK         =>  VDSO_CPUNODE_MASK
	__CPU_NUMBER_SEG      =>  __CPUNODE_SEG
	GDT_ENTRY_CPU_NUMBER  =>  GDT_ENTRY_CPUNODE
	vdso_encode_cpu_node  =>  vdso_encode_cpunode
	vdso_read_cpu_node    =>  vdso_read_cpunode

This, beyond being less confusing, also makes it easier to grep for all related
functionality:

  $ git grep -i cpunode arch/x86

Also, while at it, fix "return is not a function" style sloppiness in vdso_encode_cpunode().

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Markus T Metzger <markus.t.metzger@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1537312139-5580-2-git-send-email-chang.seok.bae@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Ingo Molnar 2018-10-08 10:41:59 +02:00
parent b2e2ba578e
commit 22245bdf0a
3 changed files with 14 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
arch/x86/entry/vdso/vgetcpu.c
View File

@ -13,7 +13,7 @@
notrace long
__vdso_getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *unused)
{
vdso_read_cpu_node(cpu, node);
vdso_read_cpunode(cpu, node);
return 0;
}

22
arch/x86/include/asm/segment.h
View File

@ -186,7 +186,7 @@
#define GDT_ENTRY_TLS_MIN 12
#define GDT_ENTRY_TLS_MAX 14
#define GDT_ENTRY_CPU_NUMBER 15
#define GDT_ENTRY_CPUNODE 15
/*
* Number of entries in the GDT table:
@ -206,7 +206,7 @@
#define __USER_DS (GDT_ENTRY_DEFAULT_USER_DS*8 + 3)
#define __USER32_DS __USER_DS
#define __USER_CS (GDT_ENTRY_DEFAULT_USER_CS*8 + 3)
#define __CPU_NUMBER_SEG (GDT_ENTRY_CPU_NUMBER*8 + 3)
#define __CPUNODE_SEG (GDT_ENTRY_CPUNODE*8 + 3)
#endif
@ -227,24 +227,24 @@
#ifdef CONFIG_X86_64
/* Bit size and mask of CPU number stored in the per CPU data (and TSC_AUX) */
#define VDSO_CPU_SIZE 12
#define VDSO_CPU_MASK 0xfff
#define VDSO_CPUNODE_BITS 12
#define VDSO_CPUNODE_MASK 0xfff
#ifndef __ASSEMBLY__
/* Helper functions to store/load CPU and node numbers */
static inline unsigned long vdso_encode_cpu_node(int cpu, unsigned long node)
static inline unsigned long vdso_encode_cpunode(int cpu, unsigned long node)
{
return ((node << VDSO_CPU_SIZE) | cpu);
return (node << VDSO_CPUNODE_BITS) | cpu;
}
static inline void vdso_read_cpu_node(unsigned *cpu, unsigned *node)
static inline void vdso_read_cpunode(unsigned *cpu, unsigned *node)
{
unsigned int p;
/*
* Load CPU and node number from GDT. LSL is faster than RDTSCP
* Load CPU and node number from the GDT. LSL is faster than RDTSCP
* and works on all CPUs. This is volatile so that it orders
* correctly with respect to barrier() and to keep GCC from cleverly
* hoisting it out of the calling function.
@ -254,12 +254,12 @@ static inline void vdso_read_cpu_node(unsigned *cpu, unsigned *node)
alternative_io ("lsl %[seg],%[p]",
".byte 0xf3,0x0f,0xc7,0xf8", /* RDPID %eax/rax */
X86_FEATURE_RDPID,
[p] "=a" (p), [seg] "r" (__CPU_NUMBER_SEG));
[p] "=a" (p), [seg] "r" (__CPUNODE_SEG));
if (cpu)
*cpu = (p & VDSO_CPU_MASK);
*cpu = (p & VDSO_CPUNODE_MASK);
if (node)
*node = (p >> VDSO_CPU_SIZE);
*node = (p >> VDSO_CPUNODE_BITS);
}
#endif /* !__ASSEMBLY__ */

4
arch/x86/kernel/cpu/common.c
View File

@ -1672,7 +1672,7 @@ static void wait_for_master_cpu(int cpu)
#ifdef CONFIG_X86_64
static void setup_getcpu(int cpu)
{
unsigned long cpudata = vdso_encode_cpu_node(cpu, early_cpu_to_node(cpu));
unsigned long cpudata = vdso_encode_cpunode(cpu, early_cpu_to_node(cpu));
struct desc_struct d = { };
if (static_cpu_has(X86_FEATURE_RDTSCP))
@ -1688,7 +1688,7 @@ static void setup_getcpu(int cpu)
d.p = 1; /* Present */
d.d = 1; /* 32-bit */
write_gdt_entry(get_cpu_gdt_rw(cpu), GDT_ENTRY_CPU_NUMBER, &d, DESCTYPE_S);
write_gdt_entry(get_cpu_gdt_rw(cpu), GDT_ENTRY_CPUNODE, &d, DESCTYPE_S);
}
#endif