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    SubjectRe: [patch V4 part 1 29/36] x86/mce: Send #MC singal from task work
    On Thu, May 07, 2020 at 11:02:09AM -0700, Andy Lutomirski wrote:
    > On Tue, May 5, 2020 at 7:13 AM Thomas Gleixner <tglx@linutronix.de> wrote:
    > >
    > > From: Peter Zijlstra <peterz@infradead.org>
    > >
    > > Convert #MC over to using task_work_add(); it will run the same code
    > > slightly later, on the return to user path of the same exception.
    >
    > I think this patch is correct, but I think it's only one small and not
    > that obviously wrong step away from being broken:
    >
    > > if ((m.cs & 3) == 3) {
    > > /* If this triggers there is no way to recover. Die hard. */
    > > BUG_ON(!on_thread_stack() || !user_mode(regs));
    > > - local_irq_enable();
    > > - preempt_enable();
    > >
    > > - if (kill_it || do_memory_failure(&m))
    > > - force_sig(SIGBUS);
    > > - preempt_disable();
    > > - local_irq_disable();
    > > + current->mce_addr = m.addr;
    > > + current->mce_status = m.mcgstatus;
    > > + current->mce_kill_me.func = kill_me_maybe;
    > > + if (kill_it)
    > > + current->mce_kill_me.func = kill_me_now;
    > > + task_work_add(current, &current->mce_kill_me, true);
    >
    > This is fine if the source was CPL3, but it's not going to work if CPL
    > was 0. We don't *currently* do this from CPL0, but people keep
    > wanting to. So perhaps there should be a comment like:
    >
    > /*
    > * The #MC originated at CPL3, so we know that we will go execute the
    > task_work before returning to the offending user code.
    > */
    >
    > IOW, if we want to recover from CPL0 #MC, we will need a different mechanism.

    See part4-18's IDTRENTRY_NOIST. That will get us a clear CPL3/CPL0
    separation.

    > I also confess a certain amount of sadness that my beautiful
    > haha-not-really-atomic-here mechanism isn't being used anymore. :(

    I think we have a subtely different interpretation of 'beautiful' here.

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