Re: [PATCH 1/2] arm64: smp: Fix pseudo NMI issues w/ broken Mediatek FW

[Doug Anderson]

Hi,

On Tue, Oct 3, 2023 at 5:29 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Mon, Oct 02, 2023 at 12:16:17PM -0700, Doug Anderson wrote:
> > Hi,
> >
> > On Mon, Oct 2, 2023 at 10:24 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > >
> > > On Mon, Oct 02, 2023 at 09:45:29AM -0700, Douglas Anderson wrote:
> > > > Some mediatek devices have the property
> > > > "mediatek,broken-save-restore-fw" in their GIC. This means that,
> > > > although the hardware supports pseudo-NMI, the firmware has a bug
> > > > that blocks enabling it. When we're in this state,
> > > > system_uses_irq_prio_masking() will return true but we'll fail to
> > > > actually enable the IRQ in the GIC.
> > > >
> > > > Let's make the code handle this. We'll detect that we failed to
> > > > request an IPI as NMI and fallback to requesting it normally. Though
> > > > we expect that either all of our requests will fail or all will
> > > > succeed, it's just as cheap to keep a per-IPI bitmap and that keeps us
> > > > robust.
> > > >
> > > > Fixes: 331a1b3a836c ("arm64: smp: Add arch support for backtrace using pseudo-NMI")
> > > > Reported-by: Chen-Yu Tsai <wenst@xxxxxxxxxxxx>
> > > > Closes: https://issuetracker.google.com/issues/197061987#comment68
> > > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > > > ---
> > > >
> > > >  arch/arm64/kernel/smp.c | 19 ++++++++++++-------
> > > >  1 file changed, 12 insertions(+), 7 deletions(-)
> > >
> > > I'm not too keen on falling back here when we have no idea why the request failed.
> > >
> > > I'd prefer if we could check the `supports_pseudo_nmis` static key directly to
> > > account for the case of broken FW, e.g. as below.
> > >
> > > Mark.
> > >
> > > ---->8----
> > > From 72fdec05c64a74f21871b44c7c760bbe07cac044 Mon Sep 17 00:00:00 2001
> > > From: Mark Rutland <mark.rutland@xxxxxxx>
> > > Date: Mon, 2 Oct 2023 18:00:36 +0100
> > > Subject: [PATCH] arm64: smp: avoid NMI IPIs with broken MediaTek FW
> > >
> > > Some MediaTek devices have broken firmware which corrupts some GICR
> > > registers behind the back of the OS, and pseudo-NMIs cannot be used on
> > > these devices. For more details see commit:
> > >
> > >   44bd78dd2b8897f5 ("irqchip/gic-v3: Disable pseudo NMIs on Mediatek devices w/ firmware issues")
> > >
> > > We did not take this problem into account in commit:
> > >
> > >   331a1b3a836c0f38 ("arm64: smp: Add arch support for backtrace using pseudo-NMI")
> > >
> > > Since that commit arm64's SMP code will try to setup some IPIs as
> > > pseudo-NMIs, even on systems with broken FW. The GICv3 code will
> > > (rightly) reject attempts to request interrupts as pseudo-NMIs,
> > > resulting in boot-time failures.
> > >
> > > Avoid the problem by taking the broken FW into account when deciding to
> > > request IPIs as pseudo-NMIs. The GICv3 driver maintains a static_key
> > > named "supports_pseudo_nmis" which is false on systems with broken FW,
> > > and we can consult this within ipi_should_be_nmi().
> > >
> > > Fixes: 331a1b3a836c0f38 ("arm64: smp: Add arch support for backtrace using pseudo-NMI")
> > > Reported-by: Chen-Yu Tsai <wenst@xxxxxxxxxxxx>
> > > Closes: https://issuetracker.google.com/issues/197061987#comment68
> > > Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> > > Cc: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > > Cc: Marc Zyngier <maz@xxxxxxxxxx>
> > > ---
> > >  arch/arm64/kernel/smp.c      | 5 ++++-
> > >  drivers/irqchip/irq-gic-v3.c | 2 +-
> > >  2 files changed, 5 insertions(+), 2 deletions(-)
> >
> > Sure, this is OK w/ me as long as folks don't mind accessing the
> > global here, it's OK w/ me:
> >
> > Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> >
> > It seems to work for me, thus:
> >
> > Tested-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> >
> >
> > > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> > > index 814d9aa93b21b..061c69160f90f 100644
> > > --- a/arch/arm64/kernel/smp.c
> > > +++ b/arch/arm64/kernel/smp.c
> > > @@ -964,7 +964,10 @@ static void smp_cross_call(const struct cpumask *target, unsigned int ipinr)
> > >
> > >  static bool ipi_should_be_nmi(enum ipi_msg_type ipi)
> > >  {
> > > -       if (!system_uses_irq_prio_masking())
> > > +       DECLARE_STATIC_KEY_FALSE(supports_pseudo_nmis);
> > > +
> > > +       if (!system_uses_irq_prio_masking() ||
> > > +           !static_branch_likely(&supports_pseudo_nmis))
> >
> > One thought, actually, is whether we should actually change
> > system_uses_irq_prio_masking() to return the correct value. What do
> > you think?
>
> I don't think we should add this to system_uses_irq_prio_masking(); that's used
> by the low-level flags manipulation code that gets inlined all over the place,
> and that code will work regarldess of whether we actually use NMI priorities.
>
> If we want to avoid using PMR masking *at all* on these platforms, we'd need to
> detect that within can_use_gic_priorities() or early_enable_pseudo_nmi().

I suspect that anyone trying to use PMR masking on these systems for
any purpose will be unhappy. The issue is talked about in:

https://issuetracker.google.com/281831288

...where you can see that the firmware on these systems isn't properly
saving/restoring some registers, including GICR_IPRIORITYR.

-Doug