Re: [patch V6 04/37] x86: Make hardware latency tracing explicit

[Andy Lutomirski]

On Mon, May 18, 2020 at 1:03 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> Andy Lutomirski <luto@xxxxxxxxxx> writes:
> > On Sun, May 17, 2020 at 1:48 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >> Remember this is about ensuring that all the state is properly
> >> established before any of this instrumentation muck can happen.
> >>
> >> DR7 handling is specific to #DB and done even before nmi_enter to
> >> prevent recursion.
> >
> > So why is this change needed?
>
> We really want nmi_enter() to be the carefully crafted mechanism which
> establishes correct state in whatever strange context the exception
> hits. Not more, not less.
>
> Random instrumentation has absolutely no business there and I went a
> long way to make sure that this is enforcible by objtool.
>
> Aside of that the tracing which is contained in nmi_enter() is about
> taking timestamps for hardware latency detection. If someone runs
> hardware latency detection with active break/watchpoints then I really
> can't help it.
>

Okay.  I'll stop looking for the bug you're fixing, then.