[RFC PATCH 0/2] Avoid booting stall caused by

There is a 10s stall in idmap_kpti_install_ng_mappings when kernel boots
on a Ampere EMAG server.

Commit f992b4dfd58b ("arm64: kpti: Add ->enable callback to remap
swapper using nG mappings") updates the nG bit runtime if kpti is
required.

But things get worse if rodata=full in map_mem(). NO_BLOCK_MAPPINGS |
NO_CONT_MAPPINGS is required when creating pagetable mapping. Hence all
ptes are fully mapped in this case. On a Ampere EMAG server with 256G
memory(pagesize=4k), it causes the 10s stall.

After moving init_cpu_features() ahead of early_fixmap_init(), we can use
cpu_have_const_cap earlier than before. Hence we can avoid this stall
by updating arm64_use_ng_mappings.

After this patch series, it reduces the kernel boot time from 14.7s to
4.1s:
Before:
[   14.757569] Freeing initrd memory: 60752K
After:
[    4.138819] Freeing initrd memory: 60752K

Set it as RFC because I want to resolve any other points which I have
misconerned.

Jia He (2):
  arm64/cpuinfo: Move init_cpu_features() ahead of early_fixmap_init()
  arm64: kpti: Update arm64_use_ng_mappings before pagetable mapping

 arch/arm64/include/asm/cpu.h |  1 +
 arch/arm64/kernel/cpuinfo.c  | 13 ++++++++++---
 arch/arm64/kernel/setup.c    | 18 +++++++++++++-----
 arch/arm64/kernel/smp.c      |  3 +--
 4 files changed, 25 insertions(+), 10 deletions(-)

-- 
2.17.1


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There is a 10s stall in idmap_kpti_install_ng_mappings when kernel boots
on a Ampere EMAG server.

Commit f992b4dfd58b ("arm64: kpti: Add ->enable callback to remap
swapper using nG mappings") updates the nG bit runtime if kpti is required.
But things get worse if rodata=full in map_mem(). NO_BLOCK_MAPPINGS |
NO_CONT_MAPPINGS is required when creating pagetable mapping. Hence all
ptes are fully mapped in this case. On a Ampere EMAG server with 256G
memory(pagesize=4k), it causes the 10s stall.

After previous commit moving init_cpu_features(), we can use
cpu_have_const_cap earlier than before. Hence we can avoid this stall
by updating arm64_use_ng_mappings.

Signed-off-by: Jia He <redacted>
---
 arch/arm64/kernel/setup.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index e078ab068f3b..51098ceb7159 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c

@@ -306,6 +306,10 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
 	/* Init the cpu feature codes for boot cpu */
 	cpuinfo_store_boot_cpu();
 
+	/* ARM64_UNMAP_KERNEL_AT_EL0 cap can be updated in cpuinfo_store_boot_cpu() */
+	if (!arm64_use_ng_mappings)
+		arm64_use_ng_mappings = cpus_have_const_cap(ARM64_UNMAP_KERNEL_AT_EL0);
+
 	early_fixmap_init();
 	early_ioremap_init();
 

-- 
2.17.1


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Move init_cpu_features() ahead of setup_arch()->early_fixmap_init(), which
is the preparation work for checking the condition to assign
arm64_use_ng_mappings as cpus_have_const_cap(ARM64_UNMAP_KERNEL_AT_EL0).

Besides, jump_label_init() is also moved ahead because
cpus_have_const_cap() depends on static key enable api.

Percpu helpers should be avoided in cpuinfo_store_boot_cpu() before percpu
init at main.c::setup_per_cpu_areas()

Signed-off-by: Jia He <redacted>
---
 arch/arm64/include/asm/cpu.h |  1 +
 arch/arm64/kernel/cpuinfo.c  | 13 ++++++++++---
 arch/arm64/kernel/setup.c    | 14 +++++++++-----
 arch/arm64/kernel/smp.c      |  3 +--
 4 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/include/asm/cpu.h b/arch/arm64/include/asm/cpu.h
index 7faae6ff3ab4..59f36f5e3c04 100644
--- a/arch/arm64/include/asm/cpu.h
+++ b/arch/arm64/include/asm/cpu.h

@@ -63,6 +63,7 @@ DECLARE_PER_CPU(struct cpuinfo_arm64, cpu_data);
 
 void cpuinfo_store_cpu(void);
 void __init cpuinfo_store_boot_cpu(void);
+void __init save_boot_cpuinfo_data(void);
 
 void __init init_cpu_features(struct cpuinfo_arm64 *info);
 void update_cpu_features(int cpu, struct cpuinfo_arm64 *info,

diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
index 77605aec25fe..f8de5b8bae20 100644
--- a/arch/arm64/kernel/cpuinfo.c
+++ b/arch/arm64/kernel/cpuinfo.c

@@ -413,9 +413,16 @@ void cpuinfo_store_cpu(void)
 
 void __init cpuinfo_store_boot_cpu(void)
 {
-	struct cpuinfo_arm64 *info = &per_cpu(cpu_data, 0);
-	__cpuinfo_store_cpu(info);
+	__cpuinfo_store_cpu(&boot_cpu_data);
 
-	boot_cpu_data = *info;
 	init_cpu_features(&boot_cpu_data);
 }
+
+void __init save_boot_cpuinfo_data(void)
+{
+	struct cpuinfo_arm64 *info;
+
+	set_my_cpu_offset(per_cpu_offset(smp_processor_id()));
+	info = &per_cpu(cpu_data, 0);
+	*info = boot_cpu_data;
+}

diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index 1a57a76e1cc2..e078ab068f3b 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c

@@ -297,16 +297,20 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
 	 */
 	arm64_use_ng_mappings = kaslr_requires_kpti();
 
-	early_fixmap_init();
-	early_ioremap_init();
-
-	setup_machine_fdt(__fdt_pointer);
-
 	/*
 	 * Initialise the static keys early as they may be enabled by the
 	 * cpufeature code and early parameters.
 	 */
 	jump_label_init();
+
+	/* Init the cpu feature codes for boot cpu */
+	cpuinfo_store_boot_cpu();
+
+	early_fixmap_init();
+	early_ioremap_init();
+
+	setup_machine_fdt(__fdt_pointer);
+
 	parse_early_param();
 
 	/*

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 2499b895efea..3df1f5b1da0b 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c

@@ -449,8 +449,7 @@ void __init smp_cpus_done(unsigned int max_cpus)
 
 void __init smp_prepare_boot_cpu(void)
 {
-	set_my_cpu_offset(per_cpu_offset(smp_processor_id()));
-	cpuinfo_store_boot_cpu();
+	save_boot_cpuinfo_data();
 
 	/*
 	 * We now know enough about the boot CPU to apply the

-- 
2.17.1


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On Wed, Jan 13, 2021 at 09:40:46AM +0800, Jia He wrote:

I don't think your patch changes this, but afaict jump_label_init() uses
per-cpu variables via cpus_read_lock(), yet we don't initialise our offset
until later on. Any idea how that works?

Will

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On Tue, Jan 26, 2021 at 01:57:13PM +0000, Will Deacon wrote:

quoted

@@ -297,16 +297,20 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
 	 */
 	arm64_use_ng_mappings = kaslr_requires_kpti();
 
-	early_fixmap_init();
-	early_ioremap_init();
-
-	setup_machine_fdt(__fdt_pointer);
-
 	/*
 	 * Initialise the static keys early as they may be enabled by the
 	 * cpufeature code and early parameters.
 	 */
 	jump_label_init();

I don't think your patch changes this, but afaict jump_label_init() uses
per-cpu variables via cpus_read_lock(), yet we don't initialise our offset
until later on. Any idea how that works?

We initialize the boot CPU's offset twice during boot, once before this
in smp_setup_processor_id(), and once afterwards in
smp_prepare_boot_cpu() since setup_per_cpu_areas() will allocate a new
region for CPU0.

IIUC per-cpu writes before smp_prepare_boot_cpu() are potentially dodgy
since they might be copied to other CPUs, but reads are all fine.

Mark.

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On Wed, Jan 13, 2021 at 09:40:47AM +0800, Jia He wrote:

Are you sure it's safe to run the cpu feature initialisation code this
early? For example, we haven't even parsed the command-line yet, so I think
a fair amount of stuff will break.

Of course, you could also just pass "mitigations=off" if you want your
performance back.

Will

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