linux-kernel - Re: [PATCH] x86/speculation/l1tf: suggest what to do on systems with too much RAM

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Message-ID: <20180823192547.GS29735@dhcp22.suse.cz>
Date:   Thu, 23 Aug 2018 21:25:47 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>, stable@...r.kernel.org
Subject: Re: [PATCH] x86/speculation/l1tf: suggest what to do on systems with
 too much RAM

On Thu 23-08-18 08:46:48, Andi Kleen wrote:
> On Thu, Aug 23, 2018 at 04:28:12PM +0200, Vlastimil Babka wrote:
> > Two users have reported [1] that they have an "extremely unlikely" system
> > with more than MAX_PA/2 memory and L1TF mitigation is not effective. Let's
> > make the warning more helpful by suggesting the proper mem=X kernel boot param,
> > a rough calculation of how much RAM can be lost (not precise if there's holes
> > between MAX_PA/2 and max_pfn in the e820 map) and a link to the L1TF document
> > to help decide if the mitigation is worth the unusable RAM.
> 
> I'm not sure anyone would really do that. After all you probably prefer
> your memory. And if it's really a non ECC client part they are are
> already used to to live very dangerously because their undetected RAM bit error
> rate will be significant. L1TF is probably one of your smaller problems
> in this case...

There are people who care about L1TF mitigations. I am not going to
question their motivation. In any case a hint how to make the mitigation
active again sounds more useful than something that sounds as scary as
"you are vulnerable".

-- 
Michal Hocko
SUSE Labs