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Date:   Wed, 14 Feb 2018 10:44:05 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Tim Chen <tim.c.chen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave@sr71.net>, hpa@zytor.com, tglx@linutronix.de,
        torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
        dwmw@amazon.co.uk, linux-tip-commits@vger.kernel.org,
        Arjan van de Ven <arjan@infradead.org>,
        Jerry Hoemann <jerry.hoemann@hpe.com>
Subject: Re: [PATCH] watchdog: hpwdt: Remove spinlock acquire and BIOS calls
 from NMI context
Message-ID: <20180214094404.GA18349@pd.tnic>
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+ Jerry

On Wed, Feb 14, 2018 at 10:31:59AM +0100, Ingo Molnar wrote:
> 
> * 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;
>  	}
>  
>  	/*

-- 
Regards/Gruss,
    Boris.

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