[SRU][Bionic][PATCH 1/6] x86/speculation/l1tf: Suggest what to do on systems with too much RAM

Juerg Haefliger juerg.haefliger at canonical.com
Wed Apr 10 10:10:03 UTC 2019

From: Vlastimil Babka <vbabka at suse.cz>

BugLink: https://bugs.launchpad.net/bugs/1822760

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 at suse.com>
Signed-off-by: Vlastimil Babka <vbabka at suse.cz>
Acked-by: Michal Hocko <mhocko at suse.com>
Cc: "H . Peter Anvin" <hpa at zytor.com>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Andi Kleen <ak at linux.intel.com>
Cc: Dave Hansen <dave.hansen at intel.com>
Cc: stable at vger.kernel.org
Link: https://lkml.kernel.org/r/966571f0-9d7f-43dc-92c6-a10eec7a1254@suse.cz

(cherry picked from commit 6a012288d6906fee1dbc244050ade1dafe4a9c8d)
Signed-off-by: Juerg Haefliger <juergh at canonical.com>
 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 f95eccf27771..0101bfa0876c 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1016,6 +1016,10 @@ 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 https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html might help you decide.\n");

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