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Subject: Re: [PATCH v5 2/9] watchdog/hpwdt: Remove legacy NMI sourcing.
To:     Jerry.Hoemann@hpe.com
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Wim Van Sebroeck <wim@linux-watchdog.org>,
        linux-watchdog@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        rwright@hpe.com, maurice.a.saldivar@hpe.com,
        Ingo Molnar <mingo@kernel.org>, marcus.folkesson@gmail.com,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        stable <stable@vger.kernel.org>
References: <20180226032227.14615-1-jerry.hoemann@hpe.com>
 <20180226032227.14615-3-jerry.hoemann@hpe.com>
 <CAK8P3a1A5d52g-3Wgs1xP3P=dT6K0RVjmamfqAi9vE5TT=mo3g@mail.gmail.com>
 <03b13d44-9ce3-ead8-020d-4b1b8114079d@roeck-us.net>
 <20180227010250.GC8244@anatevka.americas.hpqcorp.net>
From:   Guenter Roeck <linux@roeck-us.net>
Message-ID: <8766f2ee-428a-d72f-b0c3-e5fe68fd0a33@roeck-us.net>
Date:   Mon, 26 Feb 2018 17:29:55 -0800
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On 02/26/2018 05:02 PM, Jerry Hoemann wrote:
> On Mon, Feb 26, 2018 at 06:32:30AM -0800, Guenter Roeck wrote:
>> On 02/26/2018 06:11 AM, Arnd Bergmann wrote:
>>> On Mon, Feb 26, 2018 at 4:22 AM, Jerry Hoemann <jerry.hoemann@hpe.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@hpe.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.
>>
> 
> Generally speaking, HPE customers who run linux do so through a distro
> vendor and pick up patches from them.  But I'm sure there are some
> customers who do things differently.
> 
> The distro vendor's have their own repos and we'll work with them
> to back port patches to their code base.  So, I typically don't do a lot
> of kernel.org stable branch work.
> 
> Looks like objtool has been enhanced to find Spectre vulnerable code.
> Are the other kernel patches related to Spectre being back ported
> to stable release lines?  If yes, it probably make sense to do
> the hpwdt change as well.
> 

Spectre has been backported to v4.4 and later. I don't know about earlier kernels.

> Is just the patch removing the firmware call back wanted/needed? Or the
> whole driver rewrite?  (The older baseline don't have all the watchdog
> features that the patch set uses.)
> 

We would only want to backport this patch. The rest is really out of scope.

> Which stable baseline(s) would need to be patched?  Priority?
> 
> Who does it?  (i.e. do you want me to submit patches to the stable baseline?)
> 
We would tag the patch for stable (and submit it into v4.16-rc). Greg would
take care of the rest unless there are conflicts, in which case we get a note
telling us that a backport is needed.

Guenter