Re: [patch V4 part 1 29/36] x86/mce: Send #MC singal from task work

Peter Zijlstra
2020-05-08T08:48:53Z

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.