• Tim Chen's avatar
    x86/speculation: Use Indirect Branch Prediction Barrier in context switch · 18bf3c3e
    Tim Chen authored
    
    
    Flush indirect branches when switching into a process that marked itself
    non dumpable. This protects high value processes like gpg better,
    without having too high performance overhead.
    
    If done naïvely, we could switch to a kernel idle thread and then back
    to the original process, such as:
    
        process A -> idle -> process A
    
    In such scenario, we do not have to do IBPB here even though the process
    is non-dumpable, as we are switching back to the same process after a
    hiatus.
    
    To avoid the redundant IBPB, which is expensive, we track the last mm
    user context ID. The cost is to have an extra u64 mm context id to track
    the last mm we were using before switching to the init_mm used by idle.
    Avoiding the extra IBPB is probably worth the extra memory for this
    common scenario.
    
    For those cases where tlb_defer_switch_to_init_mm() returns true (non
    PCID), lazy tlb will defer switch to init_mm, so we will not be changing
    the mm for the process A -> idle -> process A switch. So IBPB will be
    skipped for this case.
    
    Thanks to the reviewers and Andy Lutomirski for the suggestion of
    using ctx_id which got rid of the problem of mm pointer recycling.
    Signed-off-by: default avatarTim Chen <tim.c.chen@linux.intel.com>
    Signed-off-by: default avatarDavid Woodhouse <dwmw@amazon.co.uk>
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Cc: ak@linux.intel.com
    Cc: karahmed@amazon.de
    Cc: arjan@linux.intel.com
    Cc: torvalds@linux-foundation.org
    Cc: linux@dominikbrodowski.net
    Cc: peterz@infradead.org
    Cc: bp@alien8.de
    Cc: luto@kernel.org
    Cc: pbonzini@redhat.com
    Cc: gregkh@linux-foundation.org
    Link: https://lkml.kernel.org/r/1517263487-3708-1-git-send-email-dwmw@amazon.co.uk
    18bf3c3e