Excerpts from Christophe Leroy's message of June 22, 2021 4:47 pm:

Le 22/06/2021 à 08:04, Nicholas Piggin a écrit :

quoted

The PPC_RFI_SRR_DEBUG check added by patch "powerpc/64s: avoid reloading
(H)SRR registers if they are still valid" has a few deficiencies. It
does not fix the actual problem, it's not enabled by default, and it
causes a program check interrupt which can cause more difficulties.

However there are a lot of paths which may clobber SRRs or change return
regs, and difficult to have a high confidence that all paths are covered
without wider testing.

Add a relatively low overhead always-enabled check that catches most
such cases, reports once, and fixes it so the kernel can continue.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
  arch/powerpc/kernel/interrupt.c | 58 +++++++++++++++++++++++++++++++++
  1 file changed, 58 insertions(+)

diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index 05fa3ae56e25..5920a3e8d1d5 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c

@@ -231,6 +231,56 @@ static notrace void booke_load_dbcr0(void)
  #endif
  }
  
+#include <linux/sched/debug.h> /* for show_regs */
+static void check_return_regs_valid(struct pt_regs *regs)
+{
+#ifdef CONFIG_PPC_BOOK3S_64
+	static bool warned = false;
+
+	if (regs->trap == 0x980 || regs->trap == 0xe00 || regs->trap == 0xe20 ||
+	    regs->trap == 0xe40 || regs->trap == 0xe60 || regs->trap == 0xe80 ||
+	    regs->trap == 0xea0 || regs->trap == 0xf80 || regs->trap == 0x1200 ||
+	    regs->trap == 0x1500 || regs->trap == 0x1600 || regs->trap == 0x1800) {

Can you use names defined in asm/interrupt.h instead of raw values ?
Some are already there, others can be added.

Good idea. Could go into a helper too actually.

I wanted to clean up the KVM definitions and unify them with interrupt.h 
defs but that's a bit of churn. Can I get to that in the next merge or 
so?

Thanks,
Nick