linux-kernel - Re: [PATCH RFC 1/5] cpu/speculation: Add 'cpu_spec

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Message-ID: <878swguer2.fsf@concordia.ellerman.id.au>
Date:   Fri, 12 Apr 2019 12:41:21 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Kosina <jikos@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jon Masters <jcm@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-s390@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tyler Hicks <tyhicks@...onical.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH RFC 1/5] cpu/speculation: Add 'cpu_spec_mitigations=' cmdline options

Josh Poimboeuf <jpoimboe@...hat.com> writes:
> On Wed, Apr 10, 2019 at 02:10:01PM +0200, Thomas Gleixner wrote:
>> On Wed, 10 Apr 2019, Michael Ellerman wrote:
>> > Josh Poimboeuf <jpoimboe@...hat.com> writes:
>> > 
>> > > On Fri, Apr 05, 2019 at 06:01:36PM +0200, Borislav Petkov wrote:
>> > >> Thinking about this more, we can shave off the first 4 chars and have it
>> > >> be:
>> > >> 
>> > >> spec_mitigations=
>> > >> 
>> > >> I think it is painfully clear which speculation mitigations we mean. And
>> > >> the other switches don't have "cpu_" prefixes too so...
>> > >
>> > > Sure, I'm ok with renaming it to that, if there are no objections.
>> > 
>> > What about when we have a mitigation for a non-speculation related bug :)
>> 
>> Those kind of silicon bugs are usually mitigated unconditionally.
>
> Right.
>
> But at least "mitigations=" is nice and short.  We could clarify in the
> documentation that it doesn't apply to *all* mitigations, only the ones
> which are optional and which can affect performance.
>
> And it would give us the freedom to include any future "optional"
> mitigations, spec or not.
>
> I kind of like it.  But I could go either way.

Some of the published SMT attacks are not speculation based.

And arguably we already have an optional mitigation for those, ie. nosmt.

cheers