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Subject[PATCHv7 19/19] x86/mm: Offset boot-time paging mode switching cost
Date
By this point we have functioning boot-time switching between 4- and
5-level paging mode. But naive approach comes with cost.

Numbers below are for kernel build, allmodconfig, 5 times.

CONFIG_X86_5LEVEL=n:

Performance counter stats for 'sh -c make -j100 -B -k >/dev/null' (5 runs):

17308719.892691 task-clock:u (msec) # 26.772 CPUs utilized ( +- 0.11% )
0 context-switches:u # 0.000 K/sec
0 cpu-migrations:u # 0.000 K/sec
331,993,164 page-faults:u # 0.019 M/sec ( +- 0.01% )
43,614,978,867,455 cycles:u # 2.520 GHz ( +- 0.01% )
39,371,534,575,126 stalled-cycles-frontend:u # 90.27% frontend cycles idle ( +- 0.09% )
28,363,350,152,428 instructions:u # 0.65 insn per cycle
# 1.39 stalled cycles per insn ( +- 0.00% )
6,316,784,066,413 branches:u # 364.948 M/sec ( +- 0.00% )
250,808,144,781 branch-misses:u # 3.97% of all branches ( +- 0.01% )

646.531974142 seconds time elapsed ( +- 1.15% )

CONFIG_X86_5LEVEL=y:

Performance counter stats for 'sh -c make -j100 -B -k >/dev/null' (5 runs):

17411536.780625 task-clock:u (msec) # 26.426 CPUs utilized ( +- 0.10% )
0 context-switches:u # 0.000 K/sec
0 cpu-migrations:u # 0.000 K/sec
331,868,663 page-faults:u # 0.019 M/sec ( +- 0.01% )
43,865,909,056,301 cycles:u # 2.519 GHz ( +- 0.01% )
39,740,130,365,581 stalled-cycles-frontend:u # 90.59% frontend cycles idle ( +- 0.05% )
28,363,358,997,959 instructions:u # 0.65 insn per cycle
# 1.40 stalled cycles per insn ( +- 0.00% )
6,316,784,937,460 branches:u # 362.793 M/sec ( +- 0.00% )
251,531,919,485 branch-misses:u # 3.98% of all branches ( +- 0.00% )

658.886307752 seconds time elapsed ( +- 0.92% )
The patch tries to fix the performance regression by using

!cpu_feature_enabled(X86_FEATURE_LA57) instead of pgtable_l5_enabled in
all hot code paths. These will statically patch the target code for
additional performance.

CONFIG_X86_5LEVEL=y + the patch:

Performance counter stats for 'sh -c make -j100 -B -k >/dev/null' (5 runs):

17381990.268506 task-clock:u (msec) # 26.907 CPUs utilized ( +- 0.19% )
0 context-switches:u # 0.000 K/sec
0 cpu-migrations:u # 0.000 K/sec
331,862,625 page-faults:u # 0.019 M/sec ( +- 0.01% )
43,697,726,320,051 cycles:u # 2.514 GHz ( +- 0.03% )
39,480,408,690,401 stalled-cycles-frontend:u # 90.35% frontend cycles idle ( +- 0.05% )
28,363,394,221,388 instructions:u # 0.65 insn per cycle
# 1.39 stalled cycles per insn ( +- 0.00% )
6,316,794,985,573 branches:u # 363.410 M/sec ( +- 0.00% )
251,013,232,547 branch-misses:u # 3.97% of all branches ( +- 0.01% )

645.991174661 seconds time elapsed ( +- 1.19% )

Unfortunately, this approach doesn't help with text size:

vmlinux.before .text size: 9798404
vmlinux.after .text size: 9802566

The .text section is increased by about 4k. Not sure if we can do anything
about this.

Signed-off-by: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
---
arch/x86/boot/compressed/misc.h | 5 +++++
arch/x86/entry/entry_64.S | 11 ++---------
arch/x86/include/asm/pgtable_64_types.h | 5 ++++-
arch/x86/kernel/head64.c | 9 +++++++--
arch/x86/kernel/head_64.S | 2 +-
arch/x86/mm/kasan_init_64.c | 6 ++++++
6 files changed, 25 insertions(+), 13 deletions(-)

diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 766a5211f827..5604f08aa405 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -11,6 +11,11 @@
#undef CONFIG_PARAVIRT_SPINLOCKS
#undef CONFIG_KASAN

+#ifdef CONFIG_X86_5LEVEL
+/* cpu_feature_enabled() cannot be used that early */
+#define pgtable_l5_enabled __pgtable_l5_enabled
+#endif
+
#include <linux/linkage.h>
#include <linux/screen_info.h>
#include <linux/elf.h>
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index eec0ca064c67..49f1e5e48b7c 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -269,15 +269,8 @@ return_from_SYSCALL_64:
* depending on paging mode) in the address.
*/
#ifdef CONFIG_X86_5LEVEL
- testl $1, pgtable_l5_enabled(%rip)
- jz 1f
- shl $(64 - 57), %rcx
- sar $(64 - 57), %rcx
- jmp 2f
-1:
- shl $(64 - 48), %rcx
- sar $(64 - 48), %rcx
-2:
+ ALTERNATIVE "shl $(64 - 48), %rcx; sar $(64 - 48), %rcx", \
+ "shl $(64 - 57), %rcx; sar $(64 - 57), %rcx", X86_FEATURE_LA57
#else
shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
index fa9f8b6592fa..0efb46fa1052 100644
--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -20,7 +20,10 @@ typedef unsigned long pgprotval_t;
typedef struct { pteval_t pte; } pte_t;

#ifdef CONFIG_X86_5LEVEL
-extern unsigned int pgtable_l5_enabled;
+extern unsigned int __pgtable_l5_enabled;
+#ifndef pgtable_l5_enabled
+#define pgtable_l5_enabled (cpu_feature_enabled(X86_FEATURE_LA57))
+#endif
#else
#define pgtable_l5_enabled 0
#endif
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 617b42c9bdbb..6dcdbdf90030 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -31,6 +31,11 @@
#include <asm/microcode.h>
#include <asm/kasan.h>

+#ifdef CONFIG_X86_5LEVEL
+#undef pgtable_l5_enabled
+#define pgtable_l5_enabled __pgtable_l5_enabled
+#endif
+
/*
* Manage page tables very early on.
*/
@@ -39,8 +44,8 @@ static unsigned int __initdata next_early_pgt;
pmdval_t early_pmd_flags = __PAGE_KERNEL_LARGE & ~(_PAGE_GLOBAL | _PAGE_NX);

#ifdef CONFIG_X86_5LEVEL
-unsigned int pgtable_l5_enabled __read_mostly;
-EXPORT_SYMBOL(pgtable_l5_enabled);
+unsigned int __pgtable_l5_enabled __read_mostly;
+EXPORT_SYMBOL(__pgtable_l5_enabled);
unsigned int pgdir_shift __read_mostly = 39;
EXPORT_SYMBOL(pgdir_shift);
unsigned int ptrs_per_p4d __read_mostly = 1;
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index e137f2665fc2..8a1fe9b63c03 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -121,7 +121,7 @@ ENTRY(secondary_startup_64)
/* Enable PAE mode, PGE and LA57 */
movl $(X86_CR4_PAE | X86_CR4_PGE), %ecx
#ifdef CONFIG_X86_5LEVEL
- testl $1, pgtable_l5_enabled(%rip)
+ testl $1, __pgtable_l5_enabled(%rip)
jz 1f
orl $X86_CR4_LA57, %ecx
1:
diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
index 9173ce1feba0..230e4ea1d3ae 100644
--- a/arch/x86/mm/kasan_init_64.c
+++ b/arch/x86/mm/kasan_init_64.c
@@ -1,5 +1,11 @@
#define DISABLE_BRANCH_PROFILING
#define pr_fmt(fmt) "kasan: " fmt
+
+#ifdef CONFIG_X86_5LEVEL
+/* Too early to use cpu_feature_enabled() */
+#define pgtable_l5_enabled __pgtable_l5_enabled
+#endif
+
#include <linux/bootmem.h>
#include <linux/kasan.h>
#include <linux/kdebug.h>
--
2.14.1
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