Re: [PATCH v7 11/16] irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs

From: Jonathan Cameron
Date: Thu Apr 25 2024 - 06:13:34 EST

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On Thu, 25 Apr 2024 10:56:37 +0100
Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:

> On Thu, 25 Apr 2024 10:28:06 +0100
> Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:
>
> > On Wed, 24 Apr 2024 13:54:38 +0100
> > Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:
> >
> > > On Tue, 23 Apr 2024 13:01:21 +0100
> > > Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > >
> > > > On Mon, 22 Apr 2024 11:40:20 +0100,
> > > > Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Thu, 18 Apr 2024 14:54:07 +0100
> > > > > Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:
> > > > >
> > > > > > From: James Morse <james.morse@xxxxxxx>
> > > > > >
> > > > > > To support virtual CPU hotplug, ACPI has added an 'online capable' bit
> > > > > > to the MADT GICC entries. This indicates a disabled CPU entry may not
> > > > > > be possible to online via PSCI until firmware has set enabled bit in
> > > > > > _STA.
> > > > > >
> > > > > > This means that a "usable" GIC is one that is marked as either enabled,
> > > > > > or online capable. Therefore, change acpi_gicc_is_usable() to check both
> > > > > > bits. However, we need to change the test in gic_acpi_match_gicc() back
> > > > > > to testing just the enabled bit so the count of enabled distributors is
> > > > > > correct.
> > > > > >
> > > > > > What about the redistributor in the GICC entry? ACPI doesn't want to say.
> > > > > > Assume the worst: When a redistributor is described in the GICC entry,
> > > > > > but the entry is marked as disabled at boot, assume the redistributor
> > > > > > is inaccessible.
> > > > > >
> > > > > > The GICv3 driver doesn't support late online of redistributors, so this
> > > > > > means the corresponding CPU can't be brought online either. Clear the
> > > > > > possible and present bits.
> > > > > >
> > > > > > Systems that want CPU hotplug in a VM can ensure their redistributors
> > > > > > are always-on, and describe them that way with a GICR entry in the MADT.
> > > > > >
> > > > > > When mapping redistributors found via GICC entries, handle the case
> > > > > > where the arch code believes the CPU is present and possible, but it
> > > > > > does not have an accessible redistributor. Print a warning and clear
> > > > > > the present and possible bits.
> > > > > >
> > > > > > Signed-off-by: James Morse <james.morse@xxxxxxx>
> > > > > > Signed-off-by: Russell King (Oracle) <rmk+kernel@xxxxxxxxxxxxxxx>
> > > > > > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> > > > >
> > > > > +CC Marc,
> > > > >
> > > > > Whilst this has been unchanged for a long time, I'm not 100% sure
> > > > > we've specifically drawn your attention to it before now.
> > > > >
> > > > > Jonathan
> > > > >
> > > > > >
> > > > > > ---
> > > > > > v7: No Change.
> > > > > > ---
> > > > > > drivers/irqchip/irq-gic-v3.c | 21 +++++++++++++++++++--
> > > > > > include/linux/acpi.h | 3 ++-
> > > > > > 2 files changed, 21 insertions(+), 3 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> > > > > > index 10af15f93d4d..66132251c1bb 100644
> > > > > > --- a/drivers/irqchip/irq-gic-v3.c
> > > > > > +++ b/drivers/irqchip/irq-gic-v3.c
> > > > > > @@ -2363,11 +2363,25 @@ gic_acpi_parse_madt_gicc(union acpi_subtable_headers *header,
> > > > > > (struct acpi_madt_generic_interrupt *)header;
> > > > > > u32 reg = readl_relaxed(acpi_data.dist_base + GICD_PIDR2) & GIC_PIDR2_ARCH_MASK;
> > > > > > u32 size = reg == GIC_PIDR2_ARCH_GICv4 ? SZ_64K * 4 : SZ_64K * 2;
> > > > > > + int cpu = get_cpu_for_acpi_id(gicc->uid);
> > > > > > void __iomem *redist_base;
> > > > > >
> > > > > > if (!acpi_gicc_is_usable(gicc))
> > > > > > return 0;
> > > > > >
> > > > > > + /*
> > > > > > + * Capable but disabled CPUs can be brought online later. What about
> > > > > > + * the redistributor? ACPI doesn't want to say!
> > > > > > + * Virtual hotplug systems can use the MADT's "always-on" GICR entries.
> > > > > > + * Otherwise, prevent such CPUs from being brought online.
> > > > > > + */
> > > > > > + if (!(gicc->flags & ACPI_MADT_ENABLED)) {
> > > > > > + pr_warn_once("CPU %u's redistributor is inaccessible: this CPU can't be brought online\n", cpu);
> > > > > > + set_cpu_present(cpu, false);
> > > > > > + set_cpu_possible(cpu, false);
> > > > > > + return 0;
> > > > > > + }
> > > >
> > > > It seems dangerous to clear those this late in the game, given how
> > > > disconnected from the architecture code this is. Are we sure that
> > > > nothing has sampled these cpumasks beforehand?
> > >
> > > Hi Marc,
> > >
> > > Any firmware that does this is being considered as buggy already
> > > but given it is firmware and the spec doesn't say much about this,
> > > there is always the possibility.
> > >
> > > Not much happens between the point where these are setup and
> > > the point where the the gic inits and this code runs, but even if careful
> > > review showed it was fine today, it will be fragile to future changes.
> > >
> > > I'm not sure there is a huge disadvantage for such broken firmware in
> > > clearing these masks from the point of view of what is used throughout
> > > the rest of the kernel. Here I think we are just looking to prevent the CPU
> > > being onlined later.
> > >
> > > We could add a set_cpu_broken() with appropriate mask.
> > > Given this is very arm64 specific I'm not sure Rafael will be keen on
> > > us checking such a mask in the generic ACPI code, but we could check it in
> > > arch_register_cpu() and just not register the cpu if it matches.
> > > That will cover the vCPU hotplug case.
> > >
> > > Does that sounds sensible, or would you prefer something else?
> >
> > Hi Marc
> >
> > Some experiments later (faking this on a physical board - I never liked
> > CPU 120 anyway!) and using a different mask brings it's own minor pain.
> >
> > When all the rest of the CPUs are brought up cpuhp_bringup_mask() is called
> > on cpu_present_mask so we need to do a dance in there to use a temporary
> > mask with broken cpus removed. I think it makes sense to cut that out
> > at the top of the cpuhp_bringup_mask() pile of actions rather than trying
> > to paper over each actual thing that is dying... (looks like an infinite loop
> > somewhere but I haven't tracked down where yet).
> >
> > I'll spin a patch so you can see what it looks like, but my concern is
> > we are just moving the risk from early users of these masks to later cases
> > where code assumes cpu_present_mask definitely means they are present.
> > That is probably a small set of cases but not nice either.
> >
> > Looks like one of those cases where we need to pick the lesser of two evils
> > which is probably still the cpu_broken_mask approach.
> >
> > On plus side if we decide to go back to the original approach having seen
> > that I already have the code :)
> >
> > Jonathan
> >
>
> Patch on top of this series. If no one shouts before I have it ready I'll
> roll a v8 with the mask introduction as a new patch and the other changes pushed into
> appropriate patches.
>
> From 361b76f36bfb4ff74fdceca7ebf14cfa43cae4a9 Mon Sep 17 00:00:00 2001
> From: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> Date: Wed, 24 Apr 2024 17:42:49 +0100
> Subject: [PATCH] cpu: Add broken cpu mask to mark CPUs where inconsistent
> firmware means we can't start them.
>
> On ARM64, it is not currently possible to use CPUs where the GICC entry
> in ACPI specifies that it is online capable but not enabled. Only
> always enabled entries are supported.
>
> Previously if this condition was met, the present and possible cpu masks
> were cleared for the relevant cpus. However, those masks may already
> have been used by other code so this is not known to be safe.
>
> An alternative is to use an additional mask (broken) and check that
> in the subset of places where these CPUs might be onlined or the
> infrastructure to indicate this is possible created.
> Specifically in bringup_nonboot_cpus() and in arch_register_cpu().
>
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
Obviously I'd missed Marc's reply on keeping this local to gicv3.
Will give that a go.

Sorry for the noise!

Jonathan

> ---
> arch/arm64/kernel/smp.c | 3 +++
> drivers/irqchip/irq-gic-v3.c | 3 +--
> include/linux/cpumask.h | 19 +++++++++++++++++++
> kernel/cpu.c | 8 +++++++-
> 4 files changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index ccb6ad347df9..39cd6a7c40d8 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -513,6 +513,9 @@ int arch_register_cpu(int cpu)
> IS_ENABLED(CONFIG_ACPI_HOTPLUG_CPU))
> return -EPROBE_DEFER;
>
> + if (cpu_broken(cpu)) /* Inconsistent firmware - can't online */
> + return -ENODEV;
> +
> #ifdef CONFIG_ACPI_HOTPLUG_CPU
> /* For now block anything that looks like physical CPU Hotplug */
> if (invalid_logical_cpuid(cpu) || !cpu_present(cpu)) {
> diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> index 66132251c1bb..a0063eb6484d 100644
> --- a/drivers/irqchip/irq-gic-v3.c
> +++ b/drivers/irqchip/irq-gic-v3.c
> @@ -2377,8 +2377,7 @@ gic_acpi_parse_madt_gicc(union acpi_subtable_headers *header,
> */
> if (!(gicc->flags & ACPI_MADT_ENABLED)) {
> pr_warn_once("CPU %u's redistributor is inaccessible: this CPU can't be brought online\n", cpu);
> - set_cpu_present(cpu, false);
> - set_cpu_possible(cpu, false);
> + set_cpu_broken(cpu);
> return 0;
> }
>
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 4b202b94c97a..70a93ad8e590 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -96,6 +96,7 @@ static inline void set_nr_cpu_ids(unsigned int nr)
> * cpu_enabled_mask - has bit 'cpu' set iff cpu can be brought online
> * cpu_online_mask - has bit 'cpu' set iff cpu available to scheduler
> * cpu_active_mask - has bit 'cpu' set iff cpu available to migration
> + * cpu_broken_mask - has bit 'cpu' set iff the cpu should never be onlined
> *
> * If !CONFIG_HOTPLUG_CPU, present == possible, and active == online.
> *
> @@ -130,12 +131,14 @@ extern struct cpumask __cpu_enabled_mask;
> extern struct cpumask __cpu_present_mask;
> extern struct cpumask __cpu_active_mask;
> extern struct cpumask __cpu_dying_mask;
> +extern struct cpumask __cpu_broken_mask;
> #define cpu_possible_mask ((const struct cpumask *)&__cpu_possible_mask)
> #define cpu_online_mask ((const struct cpumask *)&__cpu_online_mask)
> #define cpu_enabled_mask ((const struct cpumask *)&__cpu_enabled_mask)
> #define cpu_present_mask ((const struct cpumask *)&__cpu_present_mask)
> #define cpu_active_mask ((const struct cpumask *)&__cpu_active_mask)
> #define cpu_dying_mask ((const struct cpumask *)&__cpu_dying_mask)
> +#define cpu_broken_mask ((const struct cpumask *)&__cpu_broken_mask)
>
> extern atomic_t __num_online_cpus;
>
> @@ -1073,6 +1076,12 @@ set_cpu_dying(unsigned int cpu, bool dying)
> cpumask_clear_cpu(cpu, &__cpu_dying_mask);
> }
>
> +static inline void
> +set_cpu_broken(unsigned int cpu)
> +{
> + cpumask_set_cpu(cpu, &__cpu_broken_mask);
> +}
> +
> /**
> * to_cpumask - convert a NR_CPUS bitmap to a struct cpumask *
> * @bitmap: the bitmap
> @@ -1159,6 +1168,11 @@ static inline bool cpu_dying(unsigned int cpu)
> return cpumask_test_cpu(cpu, cpu_dying_mask);
> }
>
> +static inline bool cpu_broken(unsigned int cpu)
> +{
> + return cpumask_test_cpu(cpu, cpu_broken_mask);
> +}
> +
> #else
>
> #define num_online_cpus() 1U
> @@ -1197,6 +1211,11 @@ static inline bool cpu_dying(unsigned int cpu)
> return false;
> }
>
> +static inline bool cpu_broken(unsigned int cpu)
> +{
> + return false;
> +}
> +
> #endif /* NR_CPUS > 1 */
>
> #define cpu_is_offline(cpu) unlikely(!cpu_online(cpu))
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 537099bf5d02..f8b73a11869e 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1907,12 +1907,15 @@ static inline bool cpuhp_bringup_cpus_parallel(unsigned int ncpus) { return fals
>
> void __init bringup_nonboot_cpus(unsigned int max_cpus)
> {
> + static const struct cpumask tmp_mask __initdata;
> +
> /* Try parallel bringup optimization if enabled */
> if (cpuhp_bringup_cpus_parallel(max_cpus))
> return;
>
> + cpumask_andnot(&tmp_mask, cpu_present_mask, cpu_broken_mask);
> /* Full per CPU serialized bringup */
> - cpuhp_bringup_mask(cpu_present_mask, max_cpus, CPUHP_ONLINE);
> + cpuhp_bringup_mask(&tmp_mask, max_cpus, CPUHP_ONLINE);
> }
>
> #ifdef CONFIG_PM_SLEEP_SMP
> @@ -3129,6 +3132,9 @@ EXPORT_SYMBOL(__cpu_active_mask);
> struct cpumask __cpu_dying_mask __read_mostly;
> EXPORT_SYMBOL(__cpu_dying_mask);
>
> +struct cpumask __cpu_broken_mask __ro_after_init;
> +EXPORT_SYMBOL(__cpu_broken_mask);
> +
> atomic_t __num_online_cpus __read_mostly;
> EXPORT_SYMBOL(__num_online_cpus);
>

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In reply to: Jonathan Cameron: "Re: [PATCH v7 11/16] irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs"
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