Re: Hyper-Threading Vulnerability

Arjan van de Ven
2005-05-14T16:44:48Z

On Sat, 2005-05-14 at 12:30 -0400, Lee Revell wrote:
> On Sat, 2005-05-14 at 16:23 +0100, Alan Cox wrote:
> > On Sad, 2005-05-14 at 00:38, Lee Revell wrote:
> > > Well yes but you would still have to recompile those apps. And take the
> > > big performance hit from using gettimeofday vs rdtsc. Disabling HT by
> > > default looks pretty good by comparison.
> >
> > You cannot use rdtsc for anything but rough instruction timing. The
> > timers for different processors run at different speeds on some SMP
> > systems, the timer rates vary as processors change clock rate nowdays.
> > Rdtsc may also jump dramatically on a suspend/resume.
> >
> > If the app uses rdtsc then generally speaking its terminally broken. The
> > only exception is some profiling tools.
>
> That is basically all JACK and mplayer use it for. They have RT
> constraints and the tsc is used to know if we got woken up too late and
> should just drop some frames. The developers are aware of the issues
> with rdtsc and have chosen to use it anyway because these apps need
> every ounce of CPU and cannot tolerate the overhead of gettimeofday().

then JACK is terminally broken if it doesn't have a fallback for non-
rdtsc cpus.