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

[Vlastimil Babka <vbabka@suse.cz>]

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
>