[PATCH v4] powerpc: setup_64: set up PACA before parsing device tree

[Daniel Axtens]

Currently, we set up the PACA after parsing the device tree for CPU
features. Before that, r13 contains random data, which means there is
random data in r13 while we're running the generic dt parsing code.

This random data varies depending on whether we boot through a vmlinux or a
zImage: for the vmlinux case it's usually around zero, but for zImages we
see random values like 912a72603d420015.

This is poor practice, and can also lead to difficult-to-debug crashes. For
example, when kcov is enabled, the kcov instrumentation attempts to read
preempt_count out of the current task, which goes via the PACA. This then
crashes in the zImage case.

To resolve this:

 - move the PACA setup to before the cpu feature parsing.

 - because we no longer have access to cpu feature flags in PACA setup,
   change the HV feature test in the PACA setup path to consider the actual
   value of the MSR rather than the CPU feature.

Translations get switched on once we leave early_setup, so I think we'd
already catch any other cases where the PACA or task aren't set up.

Boot tested on a P9 guest and host.

Fixes: fb0b0a73b223 ("powerpc: Enable kcov")
Cc: Andrew Donnellan <ajd at linux.ibm.com>
Cc: stable at vger.kernel.org
Reviewed-by: Andrew Donnellan <ajd at linux.ibm.com>
Suggested-by: Michael Ellerman <mpe at ellerman.id.au>
Signed-off-by: Daniel Axtens <dja at axtens.net>

---

Regarding moving the comment about printk()-safety:
I am about 75% sure that the thing that makes printk() safe is the PACA,
not the CPU features. That's what commit 24d9649574fb ("[POWERPC] Document
when printk is useable") seems to indicate, but as someone wise recently
told me, "bootstrapping is hard", so I may be totally wrong.

v4: Update commit message and clarify that the mfmsr() approach is not
     for general use. Thanks Nick Piggin.

v3: Update comment, thanks Christophe Leroy.
    Remove a comment in dt_cpu_ftrs.c that is no longer accurate - thanks
      Andrew. I think we want to retain all the code still, but I'm open to
      being told otherwise.
---
 arch/powerpc/kernel/dt_cpu_ftrs.c |  1 -
 arch/powerpc/kernel/paca.c        | 10 +++++++---
 arch/powerpc/kernel/setup_64.c    | 20 +++++++++++++++-----
 3 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
index 182b4047c1ef..36bc0d5c4f3a 100644
--- a/arch/powerpc/kernel/dt_cpu_ftrs.c
+++ b/arch/powerpc/kernel/dt_cpu_ftrs.c
@@ -139,7 +139,6 @@ static void __init cpufeatures_setup_cpu(void)
 	/* Initialize the base environment -- clear FSCR/HFSCR.  */
 	hv_mode = !!(mfmsr() & MSR_HV);
 	if (hv_mode) {
-		/* CPU_FTR_HVMODE is used early in PACA setup */
 		cur_cpu_spec->cpu_features |= CPU_FTR_HVMODE;
 		mtspr(SPRN_HFSCR, 0);
 	}
diff --git a/arch/powerpc/kernel/paca.c b/arch/powerpc/kernel/paca.c
index 949eceb254d8..0ee6308541b1 100644
--- a/arch/powerpc/kernel/paca.c
+++ b/arch/powerpc/kernel/paca.c
@@ -214,11 +214,15 @@ void setup_paca(struct paca_struct *new_paca)
 	/* On Book3E, initialize the TLB miss exception frames */
 	mtspr(SPRN_SPRG_TLB_EXFRAME, local_paca->extlb);
 #else
-	/* In HV mode, we setup both HPACA and PACA to avoid problems
+	/*
+	 * In HV mode, we setup both HPACA and PACA to avoid problems
 	 * if we do a GET_PACA() before the feature fixups have been
-	 * applied
+	 * applied.
+	 *
+	 * Normally you should test against CPU_FTR_HVMODE, but CPU features
+	 * are not yet set up when we first reach here.
 	 */
-	if (early_cpu_has_feature(CPU_FTR_HVMODE))
+	if (mfmsr() & MSR_HV)
 		mtspr(SPRN_SPRG_HPACA, local_paca);
 #endif
 	mtspr(SPRN_SPRG_PACA, local_paca);
diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index e05e6dd67ae6..2259da8e8685 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -285,18 +285,28 @@ void __init early_setup(unsigned long dt_ptr)
 
 	/* -------- printk is _NOT_ safe to use here ! ------- */
 
-	/* Try new device tree based feature discovery ... */
-	if (!dt_cpu_ftrs_init(__va(dt_ptr)))
-		/* Otherwise use the old style CPU table */
-		identify_cpu(0, mfspr(SPRN_PVR));
+	/*
+	 * Assume we're on cpu 0 for now.
+	 *
+	 * We need to load a PACA very early if we are using kcov. kcov will
+	 * call in_task() in its instrumentation, which relies on the current
+	 * task from the PACA. dt_cpu_ftrs_init is coveraged-enabled and also
+	 * calls into the coverage-enabled generic dt library.
+	 *
+	 * Set up a temporary paca. It is going to be replaced below.
+	 */
 
-	/* Assume we're on cpu 0 for now. Don't write to the paca yet! */
 	initialise_paca(&boot_paca, 0);
 	setup_paca(&boot_paca);
 	fixup_boot_paca();
 
 	/* -------- printk is now safe to use ------- */
 
+	/* Try new device tree based feature discovery ... */
+	if (!dt_cpu_ftrs_init(__va(dt_ptr)))
+		/* Otherwise use the old style CPU table */
+		identify_cpu(0, mfspr(SPRN_PVR));
+
 	/* Enable early debugging if any specified (see udbg.h) */
 	udbg_early_init();
 
-- 
2.20.1