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

Borislav Petkov
2018-02-14T09:44:40Z

+ 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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