linux-kernel - Re: [PATCH v5 2/9] watchdog/hpwdt: Remove legacy NMI sourcing.

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Date:   Mon, 26 Feb 2018 15:11:01 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Jerry Hoemann <jerry.hoemann@....com>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-watchdog@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        rwright@....com, maurice.a.saldivar@....com,
        Ingo Molnar <mingo@...nel.org>, marcus.folkesson@...il.com,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH v5 2/9] watchdog/hpwdt: Remove legacy NMI sourcing.

On Mon, Feb 26, 2018 at 4:22 AM, Jerry Hoemann <jerry.hoemann@....com> wrote:
> Gen8 and prior Proliant systems supported the "CRU" interface
> to firmware.  This interfaces allows linux to "call back" into firmware
> to source the cause of an NMI.  This feature isn't fully utilized
> as the actual source of the NMI isn't printed, the driver only
> indicates that the source couldn't be determined when the call
> fails.
>
> With the advent of Gen9, iCRU replaces the CRU. The call back
> feature is no longer available in firmware.  To be compatible and
> not attempt to call back into firmware on system not supporting CRU,
> the SMBIOS table is consulted to determine if it is safe to
> make the call back or not.
>
> This results in about half of the driver code being devoted
> to either making CRU calls or determing if it is safe to make
> CRU calls.  As noted, the driver isn't really using the results of
> the CRU calls.
>
> Furthermore, as a consequence of the Spectre security issue, the
> BIOS/EFI calls are being wrapped into Spectre-disabling section.
> Removing the call back in hpwdt_pretimeout assists in this effort.
>
> As the CRU sourcing of the NMI isn't required for handling the
> NMI and there are security concerns with making the call back, remove
> the legacy (pre Gen9) NMI sourcing and the DMI code to determine if
> the system had the CRU interface.
>
> Signed-off-by: Jerry Hoemann <jerry.hoemann@....com>

This avoids a warning in mainline kernels, so that's great:

drivers/watchdog/hpwdt.o: warning: objtool: .text+0x24: indirect call
found in RETPOLINE build

I wonder what we do about stable kernels. Are both this patch and the patch
that added the objtool warning message candidates for backports to
stable kernels?

       Arnd