[PATCH] watchdog: hpwdt: Remove spinlock acquire and BIOS calls from NMI context

[Ingo Molnar <mingo@kernel.org>]

* Tim Chen <tim.c.chen@linux.intel.com> wrote:

> Dave Hansen and I had some discussions on how to handle the nested NMI and 
> firmware calls.  We thought of using a per cpu counter to record the nesting 
> call depth and toggle IBRS appropriately for the depth 0->1 and 1->0 transition.  
> Will this change be sufficient?

Yeah, so I think the first question should be: does the firmware call from NMI 
context make sense to begin with?

Because in this particular case it does not appear to be so: the reason for the 
BIOS/firmware call appears to be to determine how we nmi_panic() after receiving 
an NMI that no other NMI handler handled: with a passive-aggressive "I don't know" 
panic message or with a slightly more informative panic message.

That's not a real value-add to users - so we can avoid all these complications by 
applying the patch below:

 drivers/watchdog/hpwdt.c | 30 ++++--------------------------
 1 file changed, 4 insertions(+), 26 deletions(-)

As a bonus the spinlock use can be removed as well.

Thanks,

	Ingo

====================>
>From b038428a739a3fcf0b9678305c131f60af7422ca Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@kernel.org>
Date: Wed, 14 Feb 2018 10:24:41 +0100
Subject: [PATCH] watchdog: hpwdt: Remove spinlock acquire and BIOS calls from
 NMI context

Taking a spinlock and calling into the firmware are problematic things to
do from NMI callbacks.

It also seems completely pointless in this particular case:

 - cmn_regs is updated by the firmware call in the hpwdt_pretimeout() NMI
   callback, but there's almost no use for it: we use it only to determine
   whether to panic with an 'unknown NMI' or a slightly more descriptive
   message.

 - cmn_regs and rom_lock is not used anywhere else, other than early detection
   of the firmware capability.

So remove rom_lock, make the cmn_regs call from the detection routine only,
and thus make the NMI callback a lot more robust.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 drivers/watchdog/hpwdt.c | 30 ++++--------------------------
 1 file changed, 4 insertions(+), 26 deletions(-)

diff --git a/drivers/watchdog/hpwdt.c b/drivers/watchdog/hpwdt.c
index f1f00dfc0e68..2544fe482fe3 100644
--- a/drivers/watchdog/hpwdt.c
+++ b/drivers/watchdog/hpwdt.c
@@ -150,9 +150,7 @@ static unsigned int hpwdt_nmi_decoding;
 static unsigned int allow_kdump = 1;
 static unsigned int is_icru;
 static unsigned int is_uefi;
-static DEFINE_SPINLOCK(rom_lock);
 static void *cru_rom_addr;
-static struct cmn_registers cmn_regs;
 
 extern asmlinkage void asminline_call(struct cmn_registers *pi86Regs,
 						unsigned long *pRomEntry);
@@ -225,6 +223,7 @@ static int cru_detect(unsigned long map_entry,
 	unsigned long physical_bios_base = 0;
 	unsigned long physical_bios_offset = 0;
 	int retval = -ENODEV;
+	struct cmn_registers cmn_regs = { };
 
 	bios32_map = ioremap(map_entry, (2 * PAGE_SIZE));
 
@@ -486,32 +485,18 @@ static int hpwdt_my_nmi(void)
  */
 static int hpwdt_pretimeout(unsigned int ulReason, struct pt_regs *regs)
 {
-	unsigned long rom_pl;
-	static int die_nmi_called;
-
 	if (!hpwdt_nmi_decoding)
 		return NMI_DONE;
 
 	if ((ulReason == NMI_UNKNOWN) && !hpwdt_my_nmi())
 		return NMI_DONE;
 
-	spin_lock_irqsave(&rom_lock, rom_pl);
-	if (!die_nmi_called && !is_icru && !is_uefi)
-		asminline_call(&cmn_regs, cru_rom_addr);
-	die_nmi_called = 1;
-	spin_unlock_irqrestore(&rom_lock, rom_pl);
-
 	if (allow_kdump)
 		hpwdt_stop();
 
-	if (!is_icru && !is_uefi) {
-		if (cmn_regs.u1.ral == 0) {
-			nmi_panic(regs, "An NMI occurred, but unable to determine source.\n");
-			return NMI_HANDLED;
-		}
-	}
-	nmi_panic(regs, "An NMI occurred. Depending on your system the reason "
-		"for the NMI is logged in any one of the following "
+	nmi_panic(regs,
+		"An NMI occurred. Depending on your system the reason "
+		"for the NMI might be logged in any one of the following "
 		"resources:\n"
 		"1. Integrated Management Log (IML)\n"
 		"2. OA Syslog\n"
@@ -744,13 +729,6 @@ static int hpwdt_init_nmi_decoding(struct pci_dev *dev)
 				HPWDT_ARCH);
 			return retval;
 		}
-
-		/*
-		* We know this is the only CRU call we need to make so lets keep as
-		* few instructions as possible once the NMI comes in.
-		*/
-		cmn_regs.u1.rah = 0x0D;
-		cmn_regs.u1.ral = 0x02;
 	}
 
 	/*