linux-kernel - Re: [PATCH RFC 11/13] iommu/vt-d: Add an irq

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Date:   Wed, 06 Dec 2023 21:15:24 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 Kernel <x86@...nel.org>,
        iommu@...ts.linux.dev, Lu Baolu <baolu.lu@...ux.intel.com>,
        kvm@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Raj Ashok <ashok.raj@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>, maz@...nel.org,
        peterz@...radead.org, seanjc@...gle.com,
        Robin Murphy <robin.murphy@....com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
Subject: Re: [PATCH RFC 11/13] iommu/vt-d: Add an irq_chip for posted MSIs

On Sat, Nov 11 2023 at 20:16, Jacob Pan wrote:
> With posted MSIs, end of interrupt is handled by the notification
> handler. Each MSI handler does not go through local APIC IRR, ISR
> processing. There's no need to do apic_eoi() in those handlers.
>
> Add a new acpi_ack_irq_no_eoi() for the posted MSI IR chip. At runtime
> the call trace looks like:
>
> __sysvec_posted_msi_notification() {
>   irq_chip_ack_parent() {
>     apic_ack_irq_no_eoi();
>   }

Huch? There is something missing here to make sense.

>   handle_irq_event() {
>     handle_irq_event_percpu() {
>        driver_handler()
>     }
>   }
>
> IO-APIC IR is excluded the from posted MSI, we need to make sure it
> still performs EOI.

We need to make the code correct and write changelogs which make
sense. This sentence makes no sense whatsoever.

What has the IO-APIC to do with posted MSIs?

It's a different interrupt chip hierarchy, no?

> diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
> index 00da6cf6b07d..ca398ee9075b 100644
> --- a/arch/x86/kernel/apic/io_apic.c
> +++ b/arch/x86/kernel/apic/io_apic.c
> @@ -1993,7 +1993,7 @@ static struct irq_chip ioapic_ir_chip __read_mostly = {
>  	.irq_startup		= startup_ioapic_irq,
>  	.irq_mask		= mask_ioapic_irq,
>  	.irq_unmask		= unmask_ioapic_irq,
> -	.irq_ack		= irq_chip_ack_parent,
> +	.irq_ack		= apic_ack_irq,

Why?

>  	.irq_eoi		= ioapic_ir_ack_level,
>  	.irq_set_affinity	= ioapic_set_affinity,
>  	.irq_retrigger		= irq_chip_retrigger_hierarchy,
> diff --git a/arch/x86/kernel/apic/vector.c b/arch/x86/kernel/apic/vector.c
> index 14fc33cfdb37..01223ac4f57a 100644
> --- a/arch/x86/kernel/apic/vector.c
> +++ b/arch/x86/kernel/apic/vector.c
> @@ -911,6 +911,11 @@ void apic_ack_irq(struct irq_data *irqd)
>  	apic_eoi();
>  }
>  
> +void apic_ack_irq_no_eoi(struct irq_data *irqd)
> +{
> +	irq_move_irq(irqd);
> +}
> +

The exact purpose of that function is to invoke irq_move_irq() which is
a completely pointless exercise for interrupts which are remapped.