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

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Message-ID: <alpine.DEB.2.21.1907152129350.1767@nanos.tec.linutronix.de>
Date:   Mon, 15 Jul 2019 21:30:12 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Uros Bizjak <ubizjak@...il.com>
cc:     Andi Kleen <ak@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: [RFC PATCH, x86]: Disable CPA cache flush for selfsnoop
 targets

On Mon, 15 Jul 2019, Uros Bizjak wrote:
> On Mon, Jul 15, 2019 at 8:41 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > On Mon, 15 Jul 2019, Andi Kleen wrote:
> > > Uros Bizjak <ubizjak@...il.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