linux-kernel - Re: [PATCH 1/1] x86/pti: Fix kernel warnings for pti= and nopti cmdline options.

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Message-ID: <b9bbb279-fa8f-0784-900f-114ce186cbb3@intel.com>
Date:   Fri, 11 Aug 2023 17:08:57 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sohil Mehta <sohil.mehta@...el.com>,
        Jo Van Bulck <jo.vanbulck@...kuleuven.be>,
        linux-kernel@...r.kernel.org, dave.hansen@...ux.intel.com,
        luto@...nel.org, peterz@...radead.org, mingo@...hat.com
Cc:     x86@...nel.org, bp@...en8.de, tglx@...utronix.de, hpa@...or.com
Subject: Re: [PATCH 1/1] x86/pti: Fix kernel warnings for pti= and nopti
 cmdline options.

On 8/11/23 16:58, Sohil Mehta wrote:
> On 8/11/2023 4:42 PM, Dave Hansen wrote:
>> On 8/11/23 16:27, Jo Van Bulck wrote:
>>> Not sure which option would best match kernel coding guidelines?
>> This sound like it's getting a bit out of hand and reaching far beyond
>> cleaning up some (mostly) harmless warnings.
>>
> I agree this doesn't have to be this complex. PTI_FORCE_AUTO is unnecessary.
> 
>> pti=auto does *not* need to override mitigations=off.
> I think only pti=on needs to override mitigations=off i.e. the User is
> saying turn off mitigations but keep PTI enabled. This should be fairly
> easy to achieve with the current enum. If it is not then it's not worth
> the hassle.

It's worth *ZERO* hassle.  The docs say:

>         mitigations=
...
>                         off
>                                 Disable all optional CPU mitigations.  This
>                                 improves system performance, but it may also
>                                 expose users to several CPU vulnerabilities.
>                                 Equivalent to: 
...
>                                                nopti [X86,PPC]

That's 100% unambiguous.

If you do "mitigations=off pti=auto", you might as well have done
"pti=auto nopti" which is nonsense.

The kernel shouldn't fall over and die, but the user gets to hold the
(undefined) pieces at this point.

Please let's not make this more complicated than it has to be.