linux-kernel - Re: [PATCH] x86/speculation/l1tf: fix off-by-one error when warning that system has too much RAM

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Message-ID: <f4996252-85ca-eccc-af62-0b8572a3ffc0@suse.cz>
Date:   Thu, 23 Aug 2018 22:20:35 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     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>,
        Michal Hocko <mhocko@...nel.org>, stable@...r.kernel.org,
        Christopher Snowhill <kode54@...il.com>,
        George Anchev <studio@...hev.net>
Subject: Re: [PATCH] x86/speculation/l1tf: fix off-by-one error when warning
 that system has too much RAM

On 8/23/18 5:44 PM, Andi Kleen wrote:
> On Thu, Aug 23, 2018 at 03:44:18PM +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. In fact
>> it's a CPU with 36bits phys limit (64GB) and 32GB memory, but due to holes
>> in the e820 map, the main region is almost 500MB over the 32GB limit:
> 
> Ah I see it's a client part with very large DIMMs and someone being
> very brave and using that much memory without ECC.

Are you trying to mock the users and diminsh their report because of that?

>>
>> [    0.000000] BIOS-e820: [mem 0x0000000100000000-0x000000081effffff] usable
>>
>> Suggestions to use 'mem=32G' to prefer L1TF mitigation while losing the 500MB
>> revealed, that there's an off-by-one error in the check in
>> l1tf_select_mitigation(). l1tf_pfn_limit() returns the last usable pfn
>> (inclusive), but it's more common and hopefully less error-prone to return the
>> first pfn that's over limit, so this patch changes that and updates the other
>> callers.
> 
> I can see the off by one, but does it really cause the user's problem?
> 
> They will be still over the limit in any case, with or without off-by-one.
> 
> So the description has nothing to do with the fix. Or do I miss something?

The off-by-one happens when 'mem=32G' is used to limit the amount
memory. It's easier to do "mem=32G" with fixed code than "mem=33554428k"
to workaround the error.

> 
> -Andi
>