[RFC PATCH 1/4] powerpc/64s: Don't init FSCR_DSCR in __init_FSCR()

__init_FSCR() was added originally in commit 2468dcf641e4 ("powerpc:
Add support for context switching the TAR register") (Feb 2013), and
only set FSCR_TAR.

At that point FSCR (Facility Status and Control Register) was not
context switched, so the setting was permanent after boot.

Later we added initialisation of FSCR_DSCR to __init_FSCR(), in commit
54c9b2253d34 ("powerpc: Set DSCR bit in FSCR setup") (Mar 2013), again
that was permanent after boot.

Then commit 2517617e0de6 ("powerpc: Fix context switch DSCR on
POWER8") (Aug 2013) added a limited context switch of FSCR, just the
FSCR_DSCR bit was context switched based on thread.dscr_inherit. That
commit said "This clears the H/FSCR DSCR bit initially", but it
didn't, it left the initialisation of FSCR_DSCR in __init_FSCR().
However the initial context switch from init_task to pid 1 would clear
FSCR_DSCR because thread.dscr_inherit was 0.

That commit also introduced the requirement that FSCR_DSCR be clear
for user processes, so that we can take the facility unavailable
interrupt in order to manage dscr_inherit.

Then in commit 152d523e6307 ("powerpc: Create context switch helpers
save_sprs() and restore_sprs()") (Dec 2015) FSCR was added to
thread_struct. However it still wasn't fully context switched, we just
took the existing value and set FSCR_DSCR if the new thread had
dscr_inherit set. FSCR was still initialised at boot to FSCR_DSCR |
FSCR_TAR, but that value was not propagated into the thread_struct, so
the initial context switch set FSCR_DSCR back to 0.

Finally commit b57bd2de8c6c ("powerpc: Improve FSCR init and context
switching") (Jun 2016) added a full context switch of the FSCR, and
added an initialisation of init_task.thread.fscr to FSCR_TAR |
FSCR_EBB, but omitted FSCR_DSCR.

The end result is that swapper runs with FSCR_DSCR set because of the
initialisation in __init_FSCR(), but no other processes do, they use
the value from init_task.thread.fscr.

Having FSCR_DSCR set for swapper allows it to access SPR 3 from
userspace, but swapper never runs userspace, so it has no useful
effect. It's also confusing to have the value initialised in two
places to two different values.

So remove FSCR_DSCR from __init_FSCR(), this at least gets us to the
point where there's a single value of FSCR, even if it's still set in
two places.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/kernel/cpu_setup_power.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/cpu_setup_power.S b/arch/powerpc/kernel/cpu_setup_power.S
index a460298c7ddb..f91ecb10d0ae 100644
--- a/arch/powerpc/kernel/cpu_setup_power.S
+++ b/arch/powerpc/kernel/cpu_setup_power.S

@@ -184,7 +184,7 @@ _GLOBAL(__restore_cpu_power9)
 
 __init_FSCR:
 	mfspr	r3,SPRN_FSCR
-	ori	r3,r3,FSCR_TAR|FSCR_DSCR|FSCR_EBB
+	ori	r3,r3,FSCR_TAR|FSCR_EBB
 	mtspr	SPRN_FSCR,r3
 	blr
 

-- 
2.25.1

At boot the FSCR is initialised via one of two paths. On most systems
it's set to a hard coded value in __init_FSCR().

On newer skiboot systems we use the device tree CPU features binding,
where firmware can tell Linux what bits to set in FSCR (and HFSCR).

In both cases the value that's configured at boot is not propagated
into the init_task.thread.fscr value prior to the initial fork of init
(pid 1), which means the value is not used by any processes other than
swapper (the idle task).

For the __init_FSCR() case this is OK, because the value in
init_task.thread.fscr is initialised to something sensible. However it
does mean that the value set in __init_FSCR() is not used other than
for swapper, which is odd and confusing.

The bigger problem is for the device tree CPU features case it
prevents firmware from setting (or clearing) FSCR bits for use by user
space. This means all existing kernels can not have features
enabled/disabled by firmware if those features require
setting/clearing FSCR bits.

We can handle both cases by saving the FSCR value into
init_task.thread.fscr after we have initialised it at boot. This fixes
the bug for device tree CPU features, and will allow us to simplify
the initialisation for the __init_FSCR() case in a future patch.

Fixes: 5a61ef74f269 ("powerpc/64s: Support new device tree binding for discovering CPU features")
Cc: stable@vger.kernel.org # v4.12+
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/kernel/prom.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 1dcf0e214a22..a74bfd09cb38 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c

@@ -685,6 +685,23 @@ static void __init tm_init(void)
 static void tm_init(void) { }
 #endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
 
+#ifdef CONFIG_PPC64
+static void __init save_fscr_to_task(void)
+{
+	/*
+	 * Ensure the init_task (pid 0, aka swapper) uses the value of FSCR we
+	 * have configured via the device tree features or via __init_FSCR().
+	 * That value will then be propagated to pid 1 (init) and all future
+	 * processes.
+	 */
+	if (early_cpu_has_feature(CPU_FTR_ARCH_207S))
+		init_task.thread.fscr = mfspr(SPRN_FSCR);
+}
+#else
+static inline void save_fscr_to_task(void) {};
+#endif
+
+
 void __init early_init_devtree(void *params)
 {
 	phys_addr_t limit;

@@ -773,6 +790,8 @@ void __init early_init_devtree(void *params)
 		BUG();
 	}
 
+	save_fscr_to_task();
+
 #if defined(CONFIG_SMP) && defined(CONFIG_PPC64)
 	/* We'll later wait for secondaries to check in; there are
 	 * NCPUS-1 non-boot CPUs  :-)

-- 
2.25.1

Since the previous commit that saves the value of FSCR configured at
boot into init_task.thread.fscr, the static initialisation in
INIT_THREAD now no longer has any effect.

So remove it.

For non DT CPU features, the end result is the same, because
__init_FSCR() is called on all CPUs that have an FSCR (Power8,
Power9), and it sets FSCR_TAR & FSCR_EBB.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/include/asm/processor.h | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index 4e53df163b92..52a67835057a 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h

@@ -302,7 +302,6 @@ struct thread_struct {
 	.ksp = INIT_SP, \
 	.addr_limit = KERNEL_DS, \
 	.fpexc_mode = 0, \
-	.fscr = FSCR_TAR | FSCR_EBB \
 }
 #endif
 

-- 
2.25.1

The device tree CPU features binding includes FSCR bit numbers which
Linux is instructed to set by firmware.

Whether that's a good idea or not, in the case of the DSCR the Linux
implementation has a hard requirement that the FSCR_DSCR bit not be
set by default. We use it to track when a process reads/writes to
DSCR, so it must be clear to begin with.

So if firmware tells us to set FSCR_DSCR we must ignore it.

Currently this does not cause a bug in our DSCR handling because the
value of FSCR that the device tree CPU features code establishes is
only used by swapper. All other tasks use the value hard coded in
init_task.thread.fscr.

However we'd like to fix that in a future commit, at which point this
will become necessary.

Fixes: 5a61ef74f269 ("powerpc/64s: Support new device tree binding for discovering CPU features")
Cc: stable@vger.kernel.org # v4.12+
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/kernel/dt_cpu_ftrs.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
index 36bc0d5c4f3a..fca4d7ff22b9 100644
--- a/arch/powerpc/kernel/dt_cpu_ftrs.c
+++ b/arch/powerpc/kernel/dt_cpu_ftrs.c

@@ -346,6 +346,14 @@ static int __init feat_enable_dscr(struct dt_cpu_feature *f)
 {
 	u64 lpcr;
 
+	/*
+	 * Linux relies on FSCR[DSCR] being clear, so that we can take the
+	 * facility unavailable interrupt and track the task's usage of DSCR.
+	 * See facility_unavailable_exception().
+	 * Clear the bit here so that feat_enable() doesn't set it.
+	 */
+	f->fscr_bit_nr = -1;
+
 	feat_enable(f);
 
 	lpcr = mfspr(SPRN_LPCR);

-- 
2.25.1

Excerpts from Michael Ellerman's message of May 28, 2020 12:58 am:

__init_FSCR() was added originally in commit 2468dcf641e4 ("powerpc:
Add support for context switching the TAR register") (Feb 2013), and
only set FSCR_TAR.

At that point FSCR (Facility Status and Control Register) was not
context switched, so the setting was permanent after boot.

Later we added initialisation of FSCR_DSCR to __init_FSCR(), in commit
54c9b2253d34 ("powerpc: Set DSCR bit in FSCR setup") (Mar 2013), again
that was permanent after boot.

Then commit 2517617e0de6 ("powerpc: Fix context switch DSCR on
POWER8") (Aug 2013) added a limited context switch of FSCR, just the
FSCR_DSCR bit was context switched based on thread.dscr_inherit. That
commit said "This clears the H/FSCR DSCR bit initially", but it
didn't, it left the initialisation of FSCR_DSCR in __init_FSCR().
However the initial context switch from init_task to pid 1 would clear
FSCR_DSCR because thread.dscr_inherit was 0.

That commit also introduced the requirement that FSCR_DSCR be clear
for user processes, so that we can take the facility unavailable
interrupt in order to manage dscr_inherit.

Then in commit 152d523e6307 ("powerpc: Create context switch helpers
save_sprs() and restore_sprs()") (Dec 2015) FSCR was added to
thread_struct. However it still wasn't fully context switched, we just
took the existing value and set FSCR_DSCR if the new thread had
dscr_inherit set. FSCR was still initialised at boot to FSCR_DSCR |
FSCR_TAR, but that value was not propagated into the thread_struct, so
the initial context switch set FSCR_DSCR back to 0.

Finally commit b57bd2de8c6c ("powerpc: Improve FSCR init and context
switching") (Jun 2016) added a full context switch of the FSCR, and
added an initialisation of init_task.thread.fscr to FSCR_TAR |
FSCR_EBB, but omitted FSCR_DSCR.

The end result is that swapper runs with FSCR_DSCR set because of the
initialisation in __init_FSCR(), but no other processes do, they use
the value from init_task.thread.fscr.

Having FSCR_DSCR set for swapper allows it to access SPR 3 from
userspace, but swapper never runs userspace, so it has no useful
effect. It's also confusing to have the value initialised in two
places to two different values.

So remove FSCR_DSCR from __init_FSCR(), this at least gets us to the
point where there's a single value of FSCR, even if it's still set in
two places.

Thanks for sorting out this mess, everything looks good to me.

Thanks,
Nick

For what it's worth I tested this series on Mambo PowerNV and it seems to 
correctly enable/disable the prefix FSCR bit based on the cpu feature so feel 
free to add:

Tested-by: Alistair Popple <redacted>

Mikey is going to test out pseries.

- Alistair

On Thursday, 28 May 2020 12:58:40 AM AEST Michael Ellerman wrote:

On Fri, 2020-05-29 at 11:24 +1000, Alistair Popple wrote:

For what it's worth I tested this series on Mambo PowerNV and it seems to 
correctly enable/disable the prefix FSCR bit based on the cpu feature so feel 
free to add:

Tested-by: Alistair Popple <redacted>

Mikey is going to test out pseries.

FWIW this worked for me in the P10 + powervm sim testing.

Tested-by: Michael Neuling <redacted>

- Alistair

On Thursday, 28 May 2020 12:58:40 AM AEST Michael Ellerman wrote:

quoted

__init_FSCR() was added originally in commit 2468dcf641e4 ("powerpc:
Add support for context switching the TAR register") (Feb 2013), and
only set FSCR_TAR.

At that point FSCR (Facility Status and Control Register) was not
context switched, so the setting was permanent after boot.

Later we added initialisation of FSCR_DSCR to __init_FSCR(), in commit
54c9b2253d34 ("powerpc: Set DSCR bit in FSCR setup") (Mar 2013), again
that was permanent after boot.

Then commit 2517617e0de6 ("powerpc: Fix context switch DSCR on
POWER8") (Aug 2013) added a limited context switch of FSCR, just the
FSCR_DSCR bit was context switched based on thread.dscr_inherit. That
commit said "This clears the H/FSCR DSCR bit initially", but it
didn't, it left the initialisation of FSCR_DSCR in __init_FSCR().
However the initial context switch from init_task to pid 1 would clear
FSCR_DSCR because thread.dscr_inherit was 0.

That commit also introduced the requirement that FSCR_DSCR be clear
for user processes, so that we can take the facility unavailable
interrupt in order to manage dscr_inherit.

Then in commit 152d523e6307 ("powerpc: Create context switch helpers
save_sprs() and restore_sprs()") (Dec 2015) FSCR was added to
thread_struct. However it still wasn't fully context switched, we just
took the existing value and set FSCR_DSCR if the new thread had
dscr_inherit set. FSCR was still initialised at boot to FSCR_DSCR |
FSCR_TAR, but that value was not propagated into the thread_struct, so
the initial context switch set FSCR_DSCR back to 0.

Finally commit b57bd2de8c6c ("powerpc: Improve FSCR init and context
switching") (Jun 2016) added a full context switch of the FSCR, and
added an initialisation of init_task.thread.fscr to FSCR_TAR |
FSCR_EBB, but omitted FSCR_DSCR.

The end result is that swapper runs with FSCR_DSCR set because of the
initialisation in __init_FSCR(), but no other processes do, they use
the value from init_task.thread.fscr.

Having FSCR_DSCR set for swapper allows it to access SPR 3 from
userspace, but swapper never runs userspace, so it has no useful
effect. It's also confusing to have the value initialised in two
places to two different values.

So remove FSCR_DSCR from __init_FSCR(), this at least gets us to the
point where there's a single value of FSCR, even if it's still set in
two places.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/kernel/cpu_setup_power.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/cpu_setup_power.S

b/arch/powerpc/kernel/cpu_setup_power.S index a460298c7ddb..f91ecb10d0ae
100644

--- a/arch/powerpc/kernel/cpu_setup_power.S
+++ b/arch/powerpc/kernel/cpu_setup_power.S

@@ -184,7 +184,7 @@ _GLOBAL(__restore_cpu_power9)

 __init_FSCR:
 	mfspr	r3,SPRN_FSCR
-	ori	r3,r3,FSCR_TAR|FSCR_DSCR|FSCR_EBB
+	ori	r3,r3,FSCR_TAR|FSCR_EBB
 	mtspr	SPRN_FSCR,r3
 	blr

On Thu, 28 May 2020 00:58:40 +1000, Michael Ellerman wrote:

__init_FSCR() was added originally in commit 2468dcf641e4 ("powerpc:
Add support for context switching the TAR register") (Feb 2013), and
only set FSCR_TAR.

At that point FSCR (Facility Status and Control Register) was not
context switched, so the setting was permanent after boot.

[...]

Applied to powerpc/next.

[1/4] powerpc/64s: Don't init FSCR_DSCR in __init_FSCR()
      https://git.kernel.org/powerpc/c/0828137e8f16721842468e33df0460044a0c588b
[2/4] powerpc/64s: Don't let DT CPU features set FSCR_DSCR
      https://git.kernel.org/powerpc/c/993e3d96fd08c3ebf7566e43be9b8cd622063e6d
[3/4] powerpc/64s: Save FSCR to init_task.thread.fscr after feature init
      https://git.kernel.org/powerpc/c/912c0a7f2b5daa3cbb2bc10f303981e493de73bd
[4/4] powerpc/64s: Don't set FSCR bits in INIT_THREAD
      https://git.kernel.org/powerpc/c/c887ef5707591e84f80271e95e99ff9fb38987b5

cheers