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 <[email protected]> Signed-off-by: Vlastimil Babka <[email protected]> Acked-by: Michal Hocko <[email protected]> Cc: "H . Peter Anvin" <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Andi Kleen <[email protected]> Cc: Dave Hansen <[email protected]> Cc: [email protected] Link: https://lkml.kernel.org/r/[email protected]
btm · Aug 24, 2018 · 6a01228 · 6a01228
1 parent b0a182f
commit 6a01228
Showing 1 changed file with 4 additions and 0 deletions.
diff --git 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 https://www.kernel.org/doc/html/latest/admin-guide/l1tf.html might help you decide.\n");
 		return;
 	}