Re: [PATCH] iommu/arm-smmu: Don't disable next-page prefetcher on devices it works on

[Robin Murphy]

On 07/10/2024 11:03 am, Pankaj Patil wrote:
> On 9/4/2024 1:59 PM, Pankaj Patil wrote:
> > On 5/17/2024 10:49 PM, Doug Anderson wrote:
> > > Hi,
> > >
> > > On Fri, May 17, 2024 at 9:37 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > > > Hi Doug,
> > > >
> > > > On Mon, May 13, 2024 at 04:13:47PM -0700, Douglas Anderson wrote:
> > > > > On sc7180 trogdor devices we get a scary warning at bootup:
> > > > >    arm-smmu 15000000.iommu:
> > > > >    Failed to disable prefetcher [errata #841119 and #826419], check ACR.CACHE_LOCK
> > > > >
> > > > > We spent some time trying to figure out how we were going to fix these
> > > > > errata and whether we needed to do a firmware update. Upon closer
> > > > > inspection, however, we realized that the errata don't apply to us.
> > > > > Specifically, the errata document says that for these errata:
> > > > > * Found in: r0p0
> > > > > * Fixed in: r2p2
> > > > >
> > > > > ...and trogdor devices appear to be running r2p4. That means that they
> > > > > are unaffected despite the scary warning.
> > > > >
> > > > > The issue is that the kernel unconditionally tries to disable the
> > > > > prefetcher even on unaffected devices and then warns when it's unable
> > > > > to.
> > > > >
> > > > > Let's change the kernel to only disable the prefetcher on affected
> > > > > devices, which will get rid of the scary warning on devices that are
> > > > > unaffected. As per the comment the prefetcher is
> > > > > "not-particularly-beneficial" but it shouldn't hurt to leave it on for
> > > > > devices where it doesn't cause problems.
> > > > >
> > > > > Fixes: f87f6e5b4539 ("iommu/arm-smmu: Warn once when the perfetcher errata patch fails to apply")
> > > > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > > > > ---
> > > > >
> > > > >   drivers/iommu/arm/arm-smmu/arm-smmu-impl.c | 21 +++++++++++++--------
> > > > >   1 file changed, 13 insertions(+), 8 deletions(-)
> > > >
> > > >
> > > > Just curious, but did you see any performance impact (good or bad) as a
> > > > result of this patch? The next-page prefetcher has always looked a little
> > > > naive to me and, with a tendency for tiny TLBs in some implementations,
> > > > there's a possibility it could do more harm than good.
> > >
> > > This patch actually makes no difference on trogdor today other than
> > > getting rid of the scary warning. Specifically on trogdor the
> > > ACR.CACHE_LOCK bit seems to be set so the kernel is unable to change
> > > the setting anyway and has never been able to. We are working on
> > > figuring out how to fix the firmware and then we have to get a
> > > firmware spin before we can really see any changes. I'll keep an eye
> > > out to see if performance numbers change when the firmware uprevs.
> > >
> > > BTW: any idea how big of a deal these errata are? We're _just_
> > > finishing a firmware uprev process and there is always pushback
> > > against kicking off a new one unless the issue is important. Given
> > > that we've been living with this issue since devices shipped I'm going
> > > to assume we don't need to rush a firmware update, but if this is
> > > really scary and needs to be addressed sooner we can figure that out.
> > >
> > > -Doug
> >
> > Receiving the warning on pre-silicon platforms as well, despite being unaffected. If merged, it will help in reducing log clutter.
> > The patch applies cleanly on the tip of linux-next, tag: next-20240904.
> Following up on the patch. Please let me know if any additional
> changes are required.

Surely at pre-silicon there's really very little excuse for not just 
fixing the firmware? Anyway, it remains the case that the real issue 
here is the message and comment being misleadingly over-specific, and I 
already sent a patch to address that[1].

Thanks,
Robin.

[1] 
https://lore.kernel.org/linux-iommu/7c426dc0-4fde-4d1e-bb91-538984bd8b59@xxxxxxx/