Re: [RFC PATCH 05/13] x86/irq: Reserve a user IPI notification vector

[Thomas Gleixner]

On Mon, Sep 13 2021 at 13:01, Sohil Mehta wrote:
> A user interrupt notification vector is used on the receiver's cpu to
> identify an interrupt as a user interrupt (and not a kernel interrupt).
> Hardware uses the same notification vector to generate an IPI from a
> sender's cpu core when the SENDUIPI instruction is executed.
>
> Typically, the kernel shouldn't receive an interrupt with this vector.
> However, it is possible that the kernel might receive this vector.
>
> Scenario that can cause the spurious interrupt:
>
> Step	cpu 0 (receiver task)		cpu 1 (sender task)
> ----	---------------------		-------------------
> 1	task is running
> 2					executes SENDUIPI
> 3					IPI sent
> 4	context switched out
> 5	IPI delivered
> 	(kernel interrupt detected)
>
> A kernel interrupt can be detected, if a receiver task gets scheduled
> out after the SENDUIPI-based IPI was sent but before the IPI was
> delivered.

What happens if the SENDUIPI is issued when the target task is not on
the CPU? How is that any different from the above?

> The kernel doesn't need to do anything in this case other than receiving
> the interrupt and clearing the local APIC. The user interrupt is always
> stored in the receiver's UPID before the IPI is generated. When the
> receiver gets scheduled back the interrupt would be delivered based on
> its UPID.

So why on earth is that vector reaching the CPU at all?

> +#ifdef CONFIG_X86_USER_INTERRUPTS
> +	seq_printf(p, "%*s: ", prec, "UIS");

No point in printing that when user interrupts are not available/enabled
on the system.

> +	for_each_online_cpu(j)
> +		seq_printf(p, "%10u ", irq_stats(j)->uintr_spurious_count);
> +	seq_puts(p, "  User-interrupt spurious event\n");
>  #endif
>  	return 0;
>  }
> @@ -325,6 +331,33 @@ DEFINE_IDTENTRY_SYSVEC_SIMPLE(sysvec_kvm_posted_intr_nested_ipi)
>  }
>  #endif
>  
> +#ifdef CONFIG_X86_USER_INTERRUPTS
> +/*
> + * Handler for UINTR_NOTIFICATION_VECTOR.
> + *
> + * The notification vector is used by the cpu to detect a User Interrupt. In
> + * the typical usage, the cpu would handle this interrupt and clear the local
> + * apic.
> + *
> + * However, it is possible that the kernel might receive this vector. This can
> + * happen if the receiver thread was running when the interrupt was sent but it
> + * got scheduled out before the interrupt was delivered. The kernel doesn't
> + * need to do anything other than clearing the local APIC. A pending user
> + * interrupt is always saved in the receiver's UPID which can be referenced
> + * when the receiver gets scheduled back.
> + *
> + * If the kernel receives a storm of these, it could mean an issue with the
> + * kernel's saving and restoring of the User Interrupt MSR state; Specifically,
> + * the notification vector bits in the IA32_UINTR_MISC_MSR.

Definitely well thought out hardware that.

Thanks,

        tglx