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. Acked-by: Michal Hocko <mhocko@xxxxxxxx> > --- > 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 -- Michal Hocko SUSE Labs
