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

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

[Jia He]

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

[RFC PATCH 1/2] arm64/cpuinfo: Move init_cpu_features() ahead of setup.c::early_fixmap_init()

[Jia He]

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 <justin.he@arm.com>
---
 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

[RFC PATCH 2/2] arm64: kpti: Update arm64_use_ng_mappings before pagetable mapping

[Jia He]

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 <justin.he@arm.com>
---
 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

Re: [RFC PATCH 1/2] arm64/cpuinfo: Move init_cpu_features() ahead of setup.c::early_fixmap_init()

[Will Deacon]

On Wed, Jan 13, 2021 at 09:40:46AM +0800, Jia He wrote:
> 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 <justin.he@arm.com>
> ---
>  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();

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

Re: [RFC PATCH 1/2] arm64/cpuinfo: Move init_cpu_features() ahead of setup.c::early_fixmap_init()

[Mark Rutland]

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

Re: [RFC PATCH 2/2] arm64: kpti: Update arm64_use_ng_mappings before pagetable mapping

[Will Deacon]

On Wed, Jan 13, 2021 at 09:40:47AM +0800, Jia He wrote:
> 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 <justin.he@arm.com>
> ---
>  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);

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