linux-kernel - [PATCH 3.16 122/131] x86/speculation/l1tf: Suggest what to do on systems with too much RAM

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Message-ID: <lsq.1538257387.505139971@decadent.org.uk>
Date:   Sat, 29 Sep 2018 22:43:07 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "H . Peter Anvin" <hpa@...or.com>,
        "Michal Hocko" <mhocko@...e.com>,
        "Andi Kleen" <ak@...ux.intel.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Dave Hansen" <dave.hansen@...el.com>
Subject: [PATCH 3.16 122/131] x86/speculation/l1tf: Suggest what to do on
 systems with too much RAM

3.16.59-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Vlastimil Babka <vbabka@...e.cz>

commit 6a012288d6906fee1dbc244050ade1dafe4a9c8d upstream.

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.

Make the warning more helpful by suggesting the proper mem=X kernel boot
parameter 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@...e.com>
Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
Acked-by: Michal Hocko <mhocko@...e.com>
Cc: "H . Peter Anvin" <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andi Kleen <ak@...ux.intel.com>
Cc: Dave Hansen <dave.hansen@...el.com>
Link: https://lkml.kernel.org/r/966571f0-9d7f-43dc-92c6-a10eec7a1254@suse.cz
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kernel/cpu/bugs.c | 4 ++++
 1 file changed, 4 insertions(+)

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -717,6 +717,10 @@ static void __init l1tf_select_mitigatio
 	half_pa = (u64)l1tf_pfn_limit() << PAGE_SHIFT;
 	if (e820_any_mapped(half_pa, ULLONG_MAX - half_pa, E820_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 https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html might help you decide.\n");
 		return;
 	}