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Date:   Sat, 23 Oct 2021 10:48:22 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH 2/3] irqchip/gic-v3-its: Postpone LPI pending table freeing and memreserve

On Fri, 22 Oct 2021 11:33:06 +0100,
Valentin Schneider <valentin.schneider@....com> wrote:
> 
> Memory used by the LPI tables have to be made persistent for kexec to have
> a chance to work, as explained in [1]. If they have been made persistent
> and we are booting into a kexec'd kernel, we also need to free the pages
> that were preemptively allocated by the new kernel for those tables.
> 
> Both of those operations currently happen during its_cpu_init(), which
> happens in a _STARTING (IOW atomic) cpuhp callback for secondary
> CPUs. efi_mem_reserve_iomem() issues a GFP_ATOMIC allocation, which
> unfortunately doesn't work under PREEMPT_RT (this ends up grabbing a
> non-raw spinlock, which can sleep under PREEMPT_RT). Similarly, freeing the
> pages ends up grabbing a sleepable spinlock.
> 
> Since the memreserve is only required by kexec, it doesn't have to be
> done so early in the secondary boot process. Issue the reservation in a new
> CPUHP_AP_ONLINE_DYN cpuhp callback, and piggy-back the page freeing on top
> of it. As kexec issues a machine_shutdown() prior to machine_kexec(), it
> will be serialized vs a CPU being plugged to life by the hotplug machinery -
> either the CPU will have been brought up and have had its redistributor's
> pending table memreserved, or it never went online and will have its table
> allocated by the new kernel.
> 
> [1]: https://lore.kernel.org/lkml/20180921195954.21574-1-marc.zyngier@arm.com/
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> ---
>  drivers/irqchip/irq-gic-v3-its.c | 65 ++++++++++++++++++++++++++++++--
>  1 file changed, 61 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index a688ed5c21e8..a6a4af59205e 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -47,6 +47,8 @@
>  #define RDISTS_FLAGS_RD_TABLES_PREALLOCATED	(1 << 1)
>  
>  #define RDIST_FLAGS_LPI_ENABLED                 BIT(0)
> +#define RDIST_FLAGS_PENDTABLE_RESERVED          BIT(1)
> +#define RDIST_FLAGS_PENDTABLE_PREALLOCATED      BIT(2)
>  
>  static u32 lpi_id_bits;
>  
> @@ -3065,15 +3067,15 @@ static void its_cpu_init_lpis(void)
>  		paddr &= GENMASK_ULL(51, 16);
>  
>  		WARN_ON(!gic_check_reserved_range(paddr, LPI_PENDBASE_SZ));
> -		its_free_pending_table(gic_data_rdist()->pend_page);
> -		gic_data_rdist()->pend_page = NULL;
> +		gic_data_rdist()->flags |=
> +			RDIST_FLAGS_PENDTABLE_RESERVED |
> +			RDIST_FLAGS_PENDTABLE_PREALLOCATED;
>  
>  		goto out;
>  	}
>  
>  	pend_page = gic_data_rdist()->pend_page;
>  	paddr = page_to_phys(pend_page);
> -	WARN_ON(gic_reserve_range(paddr, LPI_PENDBASE_SZ));
>  
>  	/* set PROPBASE */
>  	val = (gic_rdists->prop_table_pa |
> @@ -3163,7 +3165,8 @@ static void its_cpu_init_lpis(void)
>  	gic_data_rdist()->flags |= RDIST_FLAGS_LPI_ENABLED;
>  	pr_info("GICv3: CPU%d: using %s LPI pending table @%pa\n",
>  		smp_processor_id(),
> -		gic_data_rdist()->pend_page ? "allocated" : "reserved",
> +		gic_data_rdist()->flags & RDIST_FLAGS_PENDTABLE_PREALLOCATED ?
> +		"reserved" : "allocated",
>  		&paddr);
>  }
>  
> @@ -5202,6 +5205,39 @@ int its_cpu_init(void)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_EFI

Why do we need this? I can't see anything that'd be problematic even
if EFI was disabled.

> +static int its_cpu_memreserve_lpi(unsigned int cpu)
> +{
> +	struct page *pend_page = gic_data_rdist()->pend_page;
> +	phys_addr_t paddr;
> +
> +	/*
> +	 * If the pending table was pre-programmed, free the memory we
> +	 * preemptively allocated.
> +	 */
> +	if (pend_page &&
> +	    (gic_data_rdist()->flags & RDIST_FLAGS_PENDTABLE_PREALLOCATED)) {
> +		its_free_pending_table(gic_data_rdist()->pend_page);
> +		gic_data_rdist()->pend_page = NULL;
> +	}

So you set it to NULL and carry on, ending up reserving a 64kB block
at address 0 if the RESERVED flag isn't set. Can this happen at all?
If, as I suspect, it cannot happen because the two flags are always
set at the same time, why do we need two flags?

My gut feeling is that if pend_page is non-NULL and that the RESERVED
flag is set, you should free the memory and leave the building.
Otherwise, reserve the memory and set the flag. PREALLOCATED doesn't
seem to make much sense on a per-CPU basis here.

> +
> +	/*
> +	 * Did we already issue a memreserve? This could be via a previous
> +	 * invocation of this callback, or in a previous life before kexec.
> +	 */
> +	if (gic_data_rdist()->flags & RDIST_FLAGS_PENDTABLE_RESERVED)
> +		return 0;
> +
> +	gic_data_rdist()->flags |= RDIST_FLAGS_PENDTABLE_RESERVED;
> +
> +	pend_page = gic_data_rdist()->pend_page;
> +	paddr = page_to_phys(pend_page);
> +	WARN_ON(gic_reserve_range(paddr, LPI_PENDBASE_SZ));

Shouldn't the flag setting be moved here and be conditional on the
reservation success? Yes, you'll get a warning each time the CPU comes
back, but at least that'd track the state correctly.

> +
> +	return 0;
> +}
> +#endif
> +
>  static const struct of_device_id its_device_id[] = {
>  	{	.compatible	= "arm,gic-v3-its",	},
>  	{},
> @@ -5385,6 +5421,23 @@ static void __init its_acpi_probe(void)
>  static void __init its_acpi_probe(void) { }
>  #endif
>  
> +static int __init its_lpi_memreserve_init(void)
> +{
> +	int state;
> +
> +	if (!efi_enabled(EFI_CONFIG_TABLES))
> +		return 0;
> +
> +	state = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
> +				"irqchip/arm/gicv3/memreserve:online",
> +				its_cpu_memreserve_lpi,
> +				NULL);
> +	if (state < 0)
> +		return state;
> +
> +	return 0;
> +}
> +
>  int __init its_init(struct fwnode_handle *handle, struct rdists *rdists,
>  		    struct irq_domain *parent_domain)
>  {
> @@ -5412,6 +5465,10 @@ int __init its_init(struct fwnode_handle *handle, struct rdists *rdists,
>  	if (err)
>  		return err;
>  
> +	err = its_lpi_memreserve_init();
> +	if (err)
> +		return err;
> +
>  	list_for_each_entry(its, &its_nodes, entry) {
>  		has_v4 |= is_v4(its);
>  		has_v4_1 |= is_v4_1(its);

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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