Re: [PATCH] irqchip/gic-v3-its: Add early memory allocation errata

[Marc Zyngier]

On 05/10/18 13:33, Matthias Brugger wrote:
> On 05/10/2018 12:55, Marc Zyngier wrote:
> > Hi Matthias,
> >
> > On 04/10/18 23:11, Matthias Brugger wrote:
> > > Friendly reminder, if anyone has any comment on the patch :)
> > >
> > > On 9/12/18 11:52 AM, matthias.bgg@xxxxxxxxxx wrote:
> > > > From: Matthias Brugger <mbrugger@xxxxxxxx>
> > > >
> > > > Some hardware does not implement two-level page tables so that
> > > > the amount of contigious memory needed by the baser is bigger
> > > > then the zone order. This is a known problem on Cavium Thunderx
> > > > with 4K page size.
> > > >
> > > > We fix this by adding an errata which allocates the memory early
> > > > in the boot cycle, using the memblock allocator.
> > > >
> > > > Signed-off-by: Matthias Brugger <mbrugger@xxxxxxxx>
> > > > ---
> > > >  Â arch/arm64/KconfigÂÂÂÂÂÂÂÂÂÂÂÂÂÂ | 12 ++++++++
> > > >  Â arch/arm64/include/asm/cpucaps.h |Â 3 +-
> > > >  Â arch/arm64/kernel/cpu_errata.cÂÂ | 33 +++++++++++++++++++++
> > > >  Â drivers/irqchip/irq-gic-v3-its.c | 50 ++++++++++++++++++++------------
> > > >  Â 4 files changed, 79 insertions(+), 19 deletions(-)
> >
> > My only comment would be to state how much I dislike both the HW and the
> > patch... ;-) The idea that we have some erratum that depends on the page size
> > doesn't feel good at all.
>
> Well ugly HW needs ugly patches ;-)
>
> > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > > index 1b1a0e95c751..dfd9fe08f0b2 100644
> > > > --- a/arch/arm64/Kconfig
> > > > +++ b/arch/arm64/Kconfig
> > > > @@ -597,6 +597,18 @@ config QCOM_FALKOR_ERRATUM_E1041
> > > >  Â ÂÂÂÂÂÂÂ If unsure, say Y.
> > > >  Â +config CAVIUM_ALLOC_ITS_TABLE_EARLY
> > > > +ÂÂÂ bool "Cavium Thunderx: Allocate the its table early"
> > > > +ÂÂÂ default y
> > > > +ÂÂÂ depends on ARM64_4K_PAGES && FORCE_MAX_ZONEORDER < 13
> >
> > Here's a though: Why don't we ensure that FORCE_MAX_ZONEORDER is such as we
> > could always allocate the same amount of memory, no matter what the page size
> > is? That, or bump FORCE_MAX_ZONEORDER to 13 if the kernel includes support for TX1.
>
> Bumping FORCE_MAX_ZONEORDER when TX1 is supported was proposed here:
> https://patchwork.kernel.org/patch/6322281/
>
> To bring in some more history, the CMA approach ended with this discussion:
> https://patchwork.kernel.org/patch/9888041/
>
> > Any of this of course requires buy-in from the arm64 maintainers, as this is
> > quite a departure from the way things work so far.
>
> With my distribution head on, I would prefer a solution that does not change
> FORCE_MAX_ZONEORDER. That's how I came to the idea providing a third solution to
> the same problem :)

Why is that a problem? What impact does this have on your favourite distro?

Thanks,

	M.
--
Jazz is not dead. It just smells funny...