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Subject: Re: [PATCH] x86/speculation/l1tf: suggest what to do on systems with
 too much RAM
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>
 <20180823142812.7363-1-vbabka@suse.cz>
 <20180823192709.GT29735@dhcp22.suse.cz>
From:   Vlastimil Babka <vbabka@suse.cz>
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Message-ID: <966571f0-9d7f-43dc-92c6-a10eec7a1254@suse.cz>
Date:   Fri, 24 Aug 2018 09:32:00 +0200
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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:

----8<----
From 977c5db27fe35a84807850b947bc5678c4d467b3 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Thu, 23 Aug 2018 16:21:29 +0200
Subject: [PATCH] x86/speculation/l1tf: suggest what to do on systems with too
 much RAM

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
to make it effective 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>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 arch/x86/kernel/cpu/bugs.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index cb4a16292aa7..5c32b5006738 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -702,6 +702,10 @@ static void __init l1tf_select_mitigation(void)
 	half_pa = (u64)l1tf_pfn_limit() << PAGE_SHIFT;
 	if (e820__mapped_any(half_pa, ULLONG_MAX - half_pa, E820_TYPE_RAM)) {
 		pr_warn("System has more than MAX_PA/2 memory. L1TF mitigation not effective.\n");
+		pr_info("You may make it effective by booting the kernel with mem=%llu parameter.\n",
+				half_pa);
+		pr_info("However, doing so will make a part of your RAM unusable.\n");
+		pr_info("Reading Documentation/admin-guide/l1tf.rst might help you decide.\n");
 		return;
 	}
 
-- 
2.18.0