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Subject: Re: [PATCH] x86/speculation/l1tf: suggest what to do on systems with
 too much RAM
From:   Vlastimil Babka <vbabka@suse.cz>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>, stable@vger.kernel.org
References: <20180823134418.17008-1-vbabka@suse.cz>
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Message-ID: <d789bd84-a982-b8bb-ee24-6a7fcdf8221d@suse.cz>
Date:   Fri, 24 Aug 2018 12:36:34 +0200
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On 08/24/2018 09:32 AM, Vlastimil Babka wrote:
> On 08/23/2018 09:27 PM, Michal Hocko wrote:
>> On Thu 23-08-18 16:28:12, 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.
>>>
>>> [1] https://bugzilla.suse.com/show_bug.cgi?id=1105536
>>>
>>> Suggested-by: Michal Hocko <mhocko@suse.com>
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>>
>> I wouldn't bother with max_pfn-half_pa part but other than that this is
>> much more useful than the original message.
> 
> Right, and it causes build failures on some configs.
> 
>> Acked-by: Michal Hocko <mhocko@suse.com>
> 
> Thanks! Here's a v2:

Just realized that kvm printk's refer to the online version at
https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html
which should be easier for the users of distro kernels, should I change
that?