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@suse.com Cc: stable@vger.kernel.org Signed-off-by: Vlastimil Babka vbabka@suse.cz --- 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; }