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

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Message-ID: <03b13d44-9ce3-ead8-020d-4b1b8114079d@roeck-us.net>
Date:   Mon, 26 Feb 2018 06:32:30 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Arnd Bergmann <arnd@...db.de>,
        Jerry Hoemann <jerry.hoemann@....com>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        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 02/26/2018 06:11 AM, Arnd Bergmann wrote:
> 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?
> 

Makes sense to me, but it is really a bit more than a bug fix, so I'll
leave it up to Jerry/HPE to make the call in respect to hpwdt.

Guenter