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@xxxxxxxx> >> Cc: stable@xxxxxxxxxxxxxxx >> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> > > 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@xxxxxxxx> Thanks! Here's a v2: ----8<---- >From 977c5db27fe35a84807850b947bc5678c4d467b3 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka <vbabka@xxxxxxx> 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@xxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Signed-off-by: Vlastimil Babka <vbabka@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"); return; } -- 2.18.0
