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

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Message-ID: <20190405131211.GE23348@zn.tnic>
Date:   Fri, 5 Apr 2019 15:12:11 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Michael Ellerman <mpe@...erman.id.au>,
        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

On Thu, Apr 04, 2019 at 11:44:11AM -0500, Josh Poimboeuf wrote:
> Keeping track of the number of mitigations for all the CPU speculation
> bugs has become overwhelming for many users.  It's getting more and more
> complicated to decide which mitigations are needed for a given
> architecture.  Complicating matters is the fact that each arch tends to
> their own custom way to mitigate the same vulnerability.

Yap, we definitely need something like that.

> Most users fall into a few basic categories:
> 
> a) they want all mitigations off;
> 
> b) they want all reasonable mitigations on, with SMT enabled even if
>    it's vulnerable; or

Uff, "reasonable" - there's the bikeshed waiting to happen.

> c) they want all reasonable mitigations on, with SMT disabled if
>    vulnerable.
> 
> Define a set of curated, arch-independent options, each of which is an
> aggregation of existing options:
> 
> - cpu_spec_mitigations=off: Disable all mitigations.

"cpu_spec_mitigations" is too long, TBH.

Imagine yourself in a loud, noisy data center - you basically can't wait
to leave - crouched over a keyboard in an impossible position, having
to type that thing and then making a typo. Whoops, too late, already
pressed Enter. Shiiiit!

Now you have to wait at least 15 mins for the damn single-threaded added
value BIOS crap to noodle through all the cores just so you can try
again, because you just rebooted the box.

And I know, my ideas for shorter cmdline options are crazy, like

cpu_spec_mtg=

which people would say, yuck, unreadable...

Oh, I know! How about

cpu_vulns=

?

We already have /sys/devices/system/cpu/vulnerabilities so it'll be the
same as that. Less things to remember.

> - cpu_spec_mitigations=auto: [default] Enable all the default
>   mitigations, but leave SMT enabled, even if it's vulnerable.
> 
> - cpu_spec_mitigations=auto,nosmt: Enable all the default mitigations,
>   disabling SMT if needed by a mitigation.

Yah, the suboption choices make sense to me.

> 
> Currently, these options are placeholders which don't actually do
> anything.  They will be fleshed out in upcoming patches.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  .../admin-guide/kernel-parameters.txt         | 23 +++++++++++++++++++
>  include/linux/cpu.h                           |  8 +++++++
>  kernel/cpu.c                                  | 15 ++++++++++++
>  3 files changed, 46 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index c4d830003b21..ac42e510bd6e 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2544,6 +2544,29 @@
>  			in the "bleeding edge" mini2440 support kernel at
>  			http://repo.or.cz/w/linux-2.6/mini2440.git
>  
> +	cpu_spec_mitigations=
> +			[KNL] Control mitigations for CPU speculation
> +			vulnerabilities on affected CPUs.  This is a set of
> +			curated, arch-independent options, each of which is an
> +			aggregation of existing options.
> +
> +			off
> +				Disable all speculative CPU mitigations.

Alias to

cpu_vulns=make_linux_fast_again

:-P

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.