[PATCH] x86/speculation/l1tf: suggest what to do on systems with too much RAM

[Vlastimil Babka]

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>
---
 arch/x86/kernel/cpu/bugs.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index cb4a16292aa7..4b820bb6c504 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -702,6 +702,11 @@ 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 up to %llu bytes of RAM unusable.\n",
+				((u64) max_pfn << PAGE_SHIFT) - half_pa);
+		pr_info("Reading Documentation/admin-guide/l1tf.rst might help you decide.");
 		return;
 	}
 
-- 
2.18.0