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    Subject[PATCH 4.4 55/80] x86/speculation/l1tf: Suggest what to do on systems with too much RAM
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Vlastimil Babka <vbabka@suse.cz>

    commit 6a012288d6906fee1dbc244050ade1dafe4a9c8d upstream.

    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 <mhocko@suse.com>
    Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
    Acked-by: Michal Hocko <mhocko@suse.com>
    Cc: "H . Peter Anvin" <hpa@zytor.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Andi Kleen <ak@linux.intel.com>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: stable@vger.kernel.org
    Link: https://lkml.kernel.org/r/966571f0-9d7f-43dc-92c6-a10eec7a1254@suse.cz
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/kernel/cpu/bugs.c | 4 ++++
    1 file changed, 4 insertions(+)

    --- a/arch/x86/kernel/cpu/bugs.c
    +++ b/arch/x86/kernel/cpu/bugs.c
    @@ -654,6 +654,10 @@ static void __init l1tf_select_mitigatio
    half_pa = (u64)l1tf_pfn_limit() << PAGE_SHIFT;
    if (e820_any_mapped(half_pa, ULLONG_MAX - half_pa, E820_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;
    }


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