[tip:x86/urgent] x86/speculation/l1tf: Suggest what to do on systems with too much RAM

From: tip-bot for Vlastimil Babka
Date: Fri Aug 24 2018 - 03:56:47 EST

Commit-ID: 4b81eae3d37dee69231592182e1e34706f149a6e
Gitweb: https://git.kernel.org/tip/4b81eae3d37dee69231592182e1e34706f149a6e
Author: Vlastimil Babka <vbabka@xxxxxxx>
AuthorDate: Thu, 23 Aug 2018 16:21:29 +0200
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Fri, 24 Aug 2018 09:51:14 +0200

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.

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@xxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: "H . Peter Anvin" <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/966571f0-9d7f-43dc-92c6-a10eec7a1254@xxxxxxx
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");