Re: [RFC PATCH v4 20/21] iommu/vt-d: hpet: Reserve an interrupt remampping table entry for watchdog

[Thomas Gleixner]

On Thu, 23 May 2019, Ricardo Neri wrote:
> When the hardlockup detector is enabled, the function
> hld_hpet_intremapactivate_irq() activates the recently created entry
> in the interrupt remapping table via the modify_irte() functions. While
> doing this, it specifies which CPU the interrupt must target via its APIC
> ID. This function can be called every time the destination iD of the
> interrupt needs to be updated; there is no need to allocate or remove
> entries in the interrupt remapping table.

Brilliant.

> +int hld_hpet_intremap_activate_irq(struct hpet_hld_data *hdata)
> +{
> +	u32 destid = apic->calc_dest_apicid(hdata->handling_cpu);
> +	struct intel_ir_data *data;
> +
> +	data = (struct intel_ir_data *)hdata->intremap_data;
> +	data->irte_entry.dest_id = IRTE_DEST(destid);
> +	return modify_irte(&data->irq_2_iommu, &data->irte_entry);

This calls modify_irte() which does at the very beginning:

   raw_spin_lock_irqsave(&irq_2_ir_lock, flags);

How is that supposed to work from NMI context? Not to talk about the
other spinlocks which are taken in the subsequent call chain.

You cannot call in any of that code from NMI context.

The only reason why this never deadlocked in your testing is that nothing
else touched that particular iommu where the HPET hangs off concurrently.

But that's just pure luck and not design. 

Thanks,

	tglx