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

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Date: Fri, 26 Jan 2024 15:31:21 -0800
From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: 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>, 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.jun.pan@...ux.intel.com
Subject: Re: [PATCH RFC 11/13] iommu/vt-d: Add an irq_chip for posted MSIs

Hi Thomas,

On Wed, 06 Dec 2023 21:15:24 +0100, Thomas Gleixner <tglx@...utronix.de>
wrote:

> 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.
Good point, I was too focused on eoi. The trace should be like

 * __sysvec_posted_msi_notification()
 *	irq_enter();
 *		handle_edge_irq()
 *			irq_chip_ack_parent()
 *				dummy(); // No EOI
 *			handle_irq_event()
 *				driver_handler()
 *	irq_enter();
 *		handle_edge_irq()
 *			irq_chip_ack_parent()
 *				dummy(); // No EOI
 *			handle_irq_event()
 *				driver_handler()
 *	irq_enter();
 *		handle_edge_irq()
 *			irq_chip_ack_parent()
 *				dummy(); // No EOI
 *			handle_irq_event()
 *				driver_handler()
 *	apic_eoi()
 * irq_exit()

> >   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?
Right, I should not modify IOAPIC chip. Just assign posted IR chip to
device MSI/x.

> > 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?
ditto.

> 
> >  	.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.

OK, I will replace this with a dummy .irq_ack() function.
Device MSIs do not have IRQD_SETAFFINITY_PENDING set.

Thanks,

Jacob