[PATCH] powerpc/64s: dt_cpu_ftrs boot time setup option

Provide a dt_cpu_ftrs= cmdline option to disable the dt_cpu_ftrs CPU
feature discovery, and fall back to the "cputable" based version.

Also allow control of advertising unknown features to userspace and
with this parameter, and remove the clunky CONFIG option.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 10 ++++++
 arch/powerpc/Kconfig                            |  5 ---
 arch/powerpc/kernel/dt_cpu_ftrs.c               | 41 +++++++++++++++++++------
 3 files changed, 42 insertions(+), 14 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index facc20a3f962..7a7c1cc80f9f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -867,6 +867,16 @@
 
 	dscc4.setup=	[NET]
 
+	dt_cpu_ftrs=	[PPC]
+			Format: {"off" | "known"}
+			Control how the dt_cpu_ftrs device-tree binding is
+			used for CPU feature discovery and setup (if it
+			exists).
+			off: Do not use it, fall back to legacy cpu table.
+			known: Do not pass through unknown features to guests
+			or userspace, only those that the kernel is not aware
+			of.
+
 	dump_apple_properties	[X86]
 			Dump name and content of EFI device properties on
 			x86 Macs.  Useful for driver authors to determine

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 5bd868f2e813..c924710e5bc7 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -391,11 +391,6 @@ config PPC_DT_CPU_FTRS
 	  firmware provides this binding.
 	  If you're not sure say Y.
 
-config PPC_CPUFEATURES_ENABLE_UNKNOWN
-	bool "cpufeatures pass through unknown features to guest/userspace"
-	depends on PPC_DT_CPU_FTRS
-	default y
-
 config HIGHMEM
 	bool "High memory support"
 	depends on PPC32

diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
index fcc7588a96d6..050925b5b451 100644
--- a/arch/powerpc/kernel/dt_cpu_ftrs.c
+++ b/arch/powerpc/kernel/dt_cpu_ftrs.c

@@ -671,12 +671,24 @@ static struct dt_cpu_feature_match __initdata
 	{"wait-v3", feat_enable, 0},
 };
 
-/* XXX: how to configure this? Default + boot time? */
-#ifdef CONFIG_PPC_CPUFEATURES_ENABLE_UNKNOWN
-#define CPU_FEATURE_ENABLE_UNKNOWN 1
-#else
-#define CPU_FEATURE_ENABLE_UNKNOWN 0
-#endif
+static bool __initdata using_dt_cpu_ftrs = true;
+static bool __initdata dt_cpu_ftrs_enable_unknown = true;
+
+static int __init dt_cpu_ftrs_parse(char *str)
+{
+	if (!str)
+		return 0;
+
+	if (!strcmp(str, "off"))
+		using_dt_cpu_ftrs = false;
+	else if (!strcmp(str, "known"))
+		dt_cpu_ftrs_enable_unknown = false;
+	else
+		return 1;
+
+	return 0;
+}
+early_param("dt_cpu_ftrs", dt_cpu_ftrs_parse);
 
 static void __init cpufeatures_setup_start(u32 isa)
 {

@@ -707,7 +719,7 @@ static bool __init cpufeatures_process_feature(struct dt_cpu_feature *f)
 		}
 	}
 
-	if (!known && CPU_FEATURE_ENABLE_UNKNOWN) {
+	if (!known && dt_cpu_ftrs_enable_unknown) {
 		if (!feat_try_enable_unknown(f)) {
 			pr_info("not enabling: %s (unknown and unsupported by kernel)\n",
 				f->name);

@@ -766,8 +778,6 @@ static int __init fdt_find_cpu_features(unsigned long node, const char *uname,
 	return 0;
 }
 
-static bool __initdata using_dt_cpu_ftrs = false;
-
 bool __init dt_cpu_ftrs_in_use(void)
 {
 	return using_dt_cpu_ftrs;

@@ -775,6 +785,16 @@ bool __init dt_cpu_ftrs_in_use(void)
 
 bool __init dt_cpu_ftrs_init(void *fdt)
 {
+	if (!using_dt_cpu_ftrs) {
+		/*
+		 * This should never happen because this runs before
+		 * early_praam, however if the init ordering changes,
+		 * test if early_param has disabled this.
+		 */
+		return false;
+	}
+	using_dt_cpu_ftrs = false;
+
 	/* Setup and verify the FDT, if it fails we just bail */
 	if (!early_init_dt_verify(fdt))
 		return false;

@@ -1027,5 +1047,8 @@ static int __init dt_cpu_ftrs_scan_callback(unsigned long node, const char
 
 void __init dt_cpu_ftrs_scan(void)
 {
+	if (!using_dt_cpu_ftrs)
+		return;
+
 	of_scan_flat_dt(dt_cpu_ftrs_scan_callback, NULL);
 }

-- 
2.11.0

On 05/11/2017 06:24 AM, Nicholas Piggin wrote:

Provide a dt_cpu_ftrs= cmdline option to disable the dt_cpu_ftrs CPU
feature discovery, and fall back to the "cputable" based version.

This boat has already sailed, I think, but "ftrs"?  Was it too difficult to type "features"?  This seems like something that should be easy and intuitive to find and understand.

:-)

PC

Paul Clarke [off-list ref] writes:

On 05/11/2017 06:24 AM, Nicholas Piggin wrote:

quoted

Provide a dt_cpu_ftrs= cmdline option to disable the dt_cpu_ftrs CPU
feature discovery, and fall back to the "cputable" based version.

This boat has already sailed, I think, but "ftrs"?

What you think vowels grow on trees! :)

Was it too difficult to type "features"?

For the command line option we could spell out features.

But should we also expand "dt", and "cpu" ?

device_tree_central_processing_unit_features=off

:P

This seems like something that should be easy and intuitive to find
and understand.

Maybe. Ideally no one will ever use it, certainly not end users, it's
primarily intended for developers doing bring-up.

cheers

On 05/11/2017 10:46 PM, Michael Ellerman wrote:

Paul Clarke [off-list ref] writes:

quoted

On 05/11/2017 06:24 AM, Nicholas Piggin wrote:

quoted

Provide a dt_cpu_ftrs= cmdline option to disable the dt_cpu_ftrs CPU
feature discovery, and fall back to the "cputable" based version.

This boat has already sailed, I think, but "ftrs"?

What you think vowels grow on trees! :)

At least you're using lower-case, which takes less space.  ;-)

quoted

Was it too difficult to type "features"?

I see "ftrs" and think "footers".

For the command line option we could spell out features.

But should we also expand "dt", and "cpu" ?

device_tree_central_processing_unit_features=off

I'm not the biggest fan of "dt" ("devtree", anyone?), but that's certainly established.

So _that's_ what "cpu" stands for!  ;-)

:P

quoted

This seems like something that should be easy and intuitive to find
and understand.

Maybe. Ideally no one will ever use it, certainly not end users, it's
primarily intended for developers doing bring-up.

I'm a developer, and I like to be able to understand something at a glance.  (Hmm, what are "CPU footers"?)

:-P

PC

Paul Clarke [off-list ref] writes:

On 05/11/2017 10:46 PM, Michael Ellerman wrote:

quoted

Paul Clarke [off-list ref] writes:

quoted

On 05/11/2017 06:24 AM, Nicholas Piggin wrote:

quoted

Provide a dt_cpu_ftrs= cmdline option to disable the dt_cpu_ftrs CPU
feature discovery, and fall back to the "cputable" based version.

This boat has already sailed, I think, but "ftrs"?

What you think vowels grow on trees! :)

At least you're using lower-case, which takes less space.  ;-)

quoted

quoted

Was it too difficult to type "features"?

I see "ftrs" and think "footers".

"FTR" - the shouty version - has meant "feature" in the powerpc code
base since at least 2002.

So it may be a crappy abbreviation but it's at least consistent :)

cheers

Nicholas Piggin [off-list ref] writes:

Provide a dt_cpu_ftrs= cmdline option to disable the dt_cpu_ftrs CPU
feature discovery, and fall back to the "cputable" based version.

I don't think this works properly.

...
	return true;

Because this runs before early_param(), as you mention,
dt_cpu_ftrs_init() returns true, which means we skip calling
identify_cpu().

So although passing dt_cpu_ftrs=off will skip the later logic, it
doesn't cause us to call identify_cpu() in early_setup() which it
should.

In practice it works because the base CPU spec that we initialise in
dt_cpu_ftrs.c is mostly OK, and skiboot also adds the cpu-version
property which causes us to call identify_cpu() again later, but that's
all a bit fragile IMHO.

So unfortunately I think we need to add logic in dt_cpu_ftrs_init() to
look for dt_cpu_ftrs=off on the command line.

The patch below seems to work, but would appreciate more eyes on it.

cheers

diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
index d6f05e4dc328..9a560954498a 100644
--- a/arch/powerpc/kernel/dt_cpu_ftrs.c
+++ b/arch/powerpc/kernel/dt_cpu_ftrs.c

@@ -8,6 +8,7 @@
 #include <linux/export.h>
 #include <linux/init.h>
 #include <linux/jump_label.h>
+#include <linux/libfdt.h>
 #include <linux/memblock.h>
 #include <linux/printk.h>
 #include <linux/sched.h>

@@ -767,6 +770,26 @@ static void __init cpufeatures_setup_finished(void)
 		cur_cpu_spec->cpu_features, cur_cpu_spec->mmu_features);
 }
 
+static int __init disabled_on_cmdline(void)
+{
+	unsigned long root, chosen;
+	const char *p;
+
+	root = of_get_flat_dt_root();
+	chosen = of_get_flat_dt_subnode_by_name(root, "chosen");
+	if (chosen == -FDT_ERR_NOTFOUND)
+		return false;
+
+	p = of_get_flat_dt_prop(chosen, "bootargs", NULL);
+	if (!p)
+		return false;
+
+	if (strstr(p, "dt_cpu_ftrs=off"))
+		return true;
+
+	return false;
+}
+
 static int __init fdt_find_cpu_features(unsigned long node, const char *uname,
 					int depth, void *data)
 {

@@ -801,6 +826,9 @@ bool __init dt_cpu_ftrs_init(void *fdt)
 	if (!of_scan_flat_dt(fdt_find_cpu_features, NULL))
 		return false;
 
+	if (disabled_on_cmdline())
+		return false;
+
 	cpufeatures_setup_cpu();
 
 	using_dt_cpu_ftrs = true;

On Thu, 2017-05-11 at 21:24 +1000, Nicholas Piggin wrote:

I wouldn't use strcmp with user passed parameters.

Balbir Singh.

On Mon, 29 May 2017 20:29:49 +1000
Michael Ellerman [off-list ref] wrote:

Nicholas Piggin [off-list ref] writes:

quoted

Provide a dt_cpu_ftrs= cmdline option to disable the dt_cpu_ftrs CPU
feature discovery, and fall back to the "cputable" based version.  

I don't think this works properly.

quoted

diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
index fcc7588a96d6..050925b5b451 100644
--- a/arch/powerpc/kernel/dt_cpu_ftrs.c
+++ b/arch/powerpc/kernel/dt_cpu_ftrs.c

@@ -775,6 +785,16 @@ bool __init dt_cpu_ftrs_in_use(void)
 
 bool __init dt_cpu_ftrs_init(void *fdt)
 {
+	if (!using_dt_cpu_ftrs) {
+		/*
+		 * This should never happen because this runs before
+		 * early_praam, however if the init ordering changes,
+		 * test if early_param has disabled this.
+		 */
+		return false;
+	}
+	using_dt_cpu_ftrs = false;
+
 	/* Setup and verify the FDT, if it fails we just bail */
 	if (!early_init_dt_verify(fdt))
 		return false;  

...
	return true;

Because this runs before early_param(), as you mention,
dt_cpu_ftrs_init() returns true, which means we skip calling
identify_cpu().

So although passing dt_cpu_ftrs=off will skip the later logic, it
doesn't cause us to call identify_cpu() in early_setup() which it
should.

In practice it works because the base CPU spec that we initialise in
dt_cpu_ftrs.c is mostly OK, and skiboot also adds the cpu-version
property which causes us to call identify_cpu() again later, but that's
all a bit fragile IMHO.

Yeah that's a huge bug :P Good catch. Possible something would fail on
CPUs that aren't POWER8/9.

So unfortunately I think we need to add logic in dt_cpu_ftrs_init() to
look for dt_cpu_ftrs=off on the command line.

The patch below seems to work, but would appreciate more eyes on it.

I can't find a problem with it, but I don't know how this fdt/prom stuff
all fits together exactly. Why is early_cmdline_parse() using call_prom
for essentially the same thing?

Thanks,
Nick

Balbir Singh [off-list ref] writes:

On Thu, 2017-05-11 at 21:24 +1000, Nicholas Piggin wrote:

quoted

diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
index fcc7588a96d6..050925b5b451 100644
--- a/arch/powerpc/kernel/dt_cpu_ftrs.c
+++ b/arch/powerpc/kernel/dt_cpu_ftrs.c

@@ -671,12 +671,24 @@ static struct dt_cpu_feature_match __initdata
 	{"wait-v3", feat_enable, 0},
 };
 
-/* XXX: how to configure this? Default + boot time? */
-#ifdef CONFIG_PPC_CPUFEATURES_ENABLE_UNKNOWN
-#define CPU_FEATURE_ENABLE_UNKNOWN 1
-#else
-#define CPU_FEATURE_ENABLE_UNKNOWN 0
-#endif
+static bool __initdata using_dt_cpu_ftrs = true;
+static bool __initdata dt_cpu_ftrs_enable_unknown = true;
+
+static int __init dt_cpu_ftrs_parse(char *str)
+{
+	if (!str)
+		return 0;
+
+	if (!strcmp(str, "off"))
+		using_dt_cpu_ftrs = false;
+	else if (!strcmp(str, "known"))
+		dt_cpu_ftrs_enable_unknown = false;
+	else
+		return 1;
+
+	return 0;
+}
+early_param("dt_cpu_ftrs", dt_cpu_ftrs_parse);

I wouldn't use strcmp with user passed parameters.

Why not? Especially considering the generic code has already parsed the
string to find the = delimiter, and is just passing us the tail.

cheers

Nicholas Piggin [off-list ref] writes:

On Mon, 29 May 2017 20:29:49 +1000
Michael Ellerman [off-list ref] wrote:

quoted

Nicholas Piggin [off-list ref] writes:

...

quoted

In practice it works because the base CPU spec that we initialise in
dt_cpu_ftrs.c is mostly OK, and skiboot also adds the cpu-version
property which causes us to call identify_cpu() again later, but that's
all a bit fragile IMHO.

Yeah that's a huge bug :P Good catch. Possible something would fail on
CPUs that aren't POWER8/9.

Yeah I had to hack a bit to make it fail because there's a few fallbacks
that save us - but yeah on something more different it would break.
 

quoted

So unfortunately I think we need to add logic in dt_cpu_ftrs_init() to
look for dt_cpu_ftrs=off on the command line.

The patch below seems to work, but would appreciate more eyes on it.

I can't find a problem with it, but I don't know how this fdt/prom stuff
all fits together exactly. Why is early_cmdline_parse() using call_prom
for essentially the same thing?

Yeah it's a mess.

early_cmdline_parse() is in prom_init.c, which is *totally* different to
prom.c, confusing much?

prom_init runs while OpenFirmware is still alive, so it calls OF (aka.
prom) for things, whereas prom.c is part of the kernel proper and only
has the flat device tree to use.


I'll squash this in and send a v2.

cheers

On Thu, 2017-05-11 at 11:24:41 UTC, Nicholas Piggin wrote:

Provide a dt_cpu_ftrs= cmdline option to disable the dt_cpu_ftrs CPU
feature discovery, and fall back to the "cputable" based version.

Also allow control of advertising unknown features to userspace and
with this parameter, and remove the clunky CONFIG option.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/a2b05b7aa60e1e9b60faf01dfb1cca

cheers