Re: [RFC PATCH, x86]: Disable CPA cache flush for selfsnoop targets

Thomas Gleixner
2019-07-15T19:30:17Z

On Mon, 15 Jul 2019, Uros Bizjak wrote:
> On Mon, Jul 15, 2019 at 8:41 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > On Mon, 15 Jul 2019, Andi Kleen wrote:
> > > Uros Bizjak <ubizjak@gmail.com> writes:
> > >
> > > > Recent patch [1] disabled a self-snoop feature on a list of processor
> > > > models with a known errata, so we are confident that the feature
> > > > should work on remaining models also for other purposes than to speed
> > > > up MTRR programming.
> > >
> > > MTRR is very different than TLBs.
> > >
> > > >From my understanding not flushing with PAT is only safe everywhere when
> > > the memory is only used for coherent devices (like the Internal GPU on
> > > Intel CPUs). We don't have any infrastructure to track or enforce
> > > this unfortunately.
> >
> > Right, we don't know where the PAT invocation comes from and whether they
> > are safe to omit flushing the cache. The module load code would be one
> > obvious candidate.
> >
> > But unless there is some really worthwhile speedup, e.g. for boot, then
> > adding some flag to let CPA know about the safe 'no flush' operation might
> > be not worth it.
>
> For the reference, FreeBSD implements this approach, later changed to
> use pmap_invalidate_cache_range ifunc (that calls
> pmap_invalidate_cache_range_selfsnoop for targets with self-snoop
> capability) and pmap_force_invalidate_cache_range [1]. The full
> cross-referenced source is at [2].

That does not answer the question whether it's worthwhile to do that.

Thanks,

tglx