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Subject[PATCHv3 13/13] 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 p4d_folded in most
code paths. These will statically patch the target code for additional
performance.

Notable exception, where cpu_feature_enabled() wasn't used is
return_from_SYSCALL_64(). It's written in assembly and there's no easy
way to use the same approach. We can come back to this later, if
required.

Also, I had to re-write number of static inline helpers as macros.
It was needed to break header dependency loop between cpufeature.h and
pgtable_types.h.

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% )

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
arch/x86/boot/compressed/misc.h | 5 +++
arch/x86/entry/entry_64.S | 2 +-
arch/x86/include/asm/paravirt.h | 23 ++++++-----
arch/x86/include/asm/pgtable_64_types.h | 5 ++-
arch/x86/include/asm/pgtable_types.h | 67 ++++++++-------------------------
arch/x86/kernel/head64.c | 5 +++
arch/x86/kernel/head_64.S | 6 +--
arch/x86/mm/kasan_init_64.c | 6 +++
8 files changed, 53 insertions(+), 66 deletions(-)

diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 766a5211f827..28ac72acaa31 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 p4d_folded __p4d_folded
+#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 077e8b45784c..6f92e61d35ac 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -274,7 +274,7 @@ return_from_SYSCALL_64:
* depending on paging mode) in the address.
*/
#ifdef CONFIG_X86_5LEVEL
- testl $1, p4d_folded(%rip)
+ testl $1, __p4d_folded(%rip)
jnz 1f
shl $(64 - 57), %rcx
sar $(64 - 57), %rcx
diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 69c3cb792f34..77d69be89d4b 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -604,19 +604,22 @@ static inline p4dval_t p4d_val(p4d_t p4d)
return PVOP_CALLEE1(p4dval_t, pv_mmu_ops.p4d_val, p4d.p4d);
}

-static inline void set_pgd(pgd_t *pgdp, pgd_t pgd)
+static inline void __set_pgd(pgd_t *pgdp, pgd_t pgd)
{
- if (p4d_folded)
- set_p4d((p4d_t *)(pgdp), (p4d_t) { pgd.pgd });
- else
- PVOP_VCALL2(pv_mmu_ops.set_pgd, pgdp, native_pgd_val(pgd));
+ PVOP_VCALL2(pv_mmu_ops.set_pgd, pgdp, native_pgd_val(pgd));
}

-static inline void pgd_clear(pgd_t *pgdp)
-{
- if (!p4d_folded)
- set_pgd(pgdp, __pgd(0));
-}
+#define set_pgd(pgdp, pgdval) do { \
+ if (p4d_folded) \
+ set_p4d((p4d_t *)(pgdp), (p4d_t) { (pgdval).pgd }); \
+ else \
+ __set_pgd(pgdp, pgdval); \
+} while (0)
+
+#define pgd_clear(pgdp) do { \
+ if (!p4d_folded) \
+ set_pgd(pgdp, __pgd(0)); \
+} while (0)

#endif /* CONFIG_PGTABLE_LEVELS == 5 */

diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
index a29cc23e96b8..9b50cc0f82e1 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 p4d_folded;
+extern unsigned int __p4d_folded;
+#ifndef p4d_folded
+#define p4d_folded (!cpu_feature_enabled(X86_FEATURE_LA57))
+#endif
#else
#define p4d_folded 1
#endif
diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index 399261ce904c..ece3ad5215ad 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -290,10 +290,7 @@ static inline pgdval_t native_pgd_val(pgd_t pgd)
return pgd.pgd;
}

-static inline pgdval_t pgd_flags(pgd_t pgd)
-{
- return native_pgd_val(pgd) & PTE_FLAGS_MASK;
-}
+#define pgd_flags(pgd) (native_pgd_val(pgd) & PTE_FLAGS_MASK)

#if CONFIG_PGTABLE_LEVELS > 4
typedef struct { p4dval_t p4d; } p4d_t;
@@ -363,57 +360,28 @@ static inline pmdval_t native_pmd_val(pmd_t pmd)
}
#endif

-static inline p4dval_t p4d_pfn_mask(p4d_t p4d)
-{
- /* No 512 GiB huge pages yet */
- return PTE_PFN_MASK;
-}
+/* No 512 GiB huge pages yet */
+#define p4d_pfn_mask(p4d) PTE_PFN_MASK

-static inline p4dval_t p4d_flags_mask(p4d_t p4d)
-{
- return ~p4d_pfn_mask(p4d);
-}
+#define p4d_flags_mask(p4d) (~p4d_pfn_mask(p4d))

-static inline p4dval_t p4d_flags(p4d_t p4d)
-{
- return native_p4d_val(p4d) & p4d_flags_mask(p4d);
-}
+#define p4d_flags(p4d) (native_p4d_val(p4d) & p4d_flags_mask(p4d))

-static inline pudval_t pud_pfn_mask(pud_t pud)
-{
- if (native_pud_val(pud) & _PAGE_PSE)
- return PHYSICAL_PUD_PAGE_MASK;
- else
- return PTE_PFN_MASK;
-}
+#define pud_pfn_mask(pud) \
+ (native_pud_val(pud) & _PAGE_PSE ? \
+ PHYSICAL_PUD_PAGE_MASK : PTE_PFN_MASK)

-static inline pudval_t pud_flags_mask(pud_t pud)
-{
- return ~pud_pfn_mask(pud);
-}
+#define pud_flags_mask(pud) (~pud_pfn_mask(pud))

-static inline pudval_t pud_flags(pud_t pud)
-{
- return native_pud_val(pud) & pud_flags_mask(pud);
-}
+#define pud_flags(pud) (native_pud_val(pud) & pud_flags_mask(pud))

-static inline pmdval_t pmd_pfn_mask(pmd_t pmd)
-{
- if (native_pmd_val(pmd) & _PAGE_PSE)
- return PHYSICAL_PMD_PAGE_MASK;
- else
- return PTE_PFN_MASK;
-}
+#define pmd_pfn_mask(pmd) \
+ (native_pmd_val(pmd) & _PAGE_PSE ? \
+ PHYSICAL_PMD_PAGE_MASK : PTE_PFN_MASK)

-static inline pmdval_t pmd_flags_mask(pmd_t pmd)
-{
- return ~pmd_pfn_mask(pmd);
-}
+#define pmd_flags_mask(pmd) (~pmd_pfn_mask(pmd))

-static inline pmdval_t pmd_flags(pmd_t pmd)
-{
- return native_pmd_val(pmd) & pmd_flags_mask(pmd);
-}
+#define pmd_flags(pmd) (native_pmd_val(pmd) & pmd_flags_mask(pmd))

static inline pte_t native_make_pte(pteval_t val)
{
@@ -425,10 +393,7 @@ static inline pteval_t native_pte_val(pte_t pte)
return pte.pte;
}

-static inline pteval_t pte_flags(pte_t pte)
-{
- return native_pte_val(pte) & PTE_FLAGS_MASK;
-}
+#define pte_flags(pte) (native_pte_val(pte) & PTE_FLAGS_MASK)

#define pgprot_val(x) ((x).pgprot)
#define __pgprot(x) ((pgprot_t) { (x) } )
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 4c9ffb58a3b5..10736723c4e0 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 p4d_folded
+#define p4d_folded __p4d_folded
+#endif
+
/*
* Manage page tables very early on.
*/
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 9de244aa72fd..805d915f730a 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, p4d_folded(%rip)
+ testl $1, __p4d_folded(%rip)
jnz 1f
orl $X86_CR4_LA57, %ecx
1:
@@ -433,9 +433,9 @@ ENTRY(phys_base)
EXPORT_SYMBOL(phys_base)

#ifdef CONFIG_X86_5LEVEL
-ENTRY(p4d_folded)
+ENTRY(__p4d_folded)
.word 1
-EXPORT_SYMBOL(p4d_folded)
+EXPORT_SYMBOL(__p4d_folded)
#endif

#include "../../x86/xen/xen-head.S"
diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
index 17256253a887..6dfdfddba09a 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 p4d_folded __p4d_folded
+#endif
+
#include <linux/bootmem.h>
#include <linux/kasan.h>
#include <linux/kdebug.h>
--
2.13.2
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