linux-kernel - Re: [PATCH v2 10/13] x86/irq: Extend checks for pending vectors to posted interrupts

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Date: Fri, 12 Apr 2024 11:23:31 -0700
From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: LKML <linux-kernel@...r.kernel.org>, X86 Kernel <x86@...nel.org>, Peter
 Zijlstra <peterz@...radead.org>, "iommu@...ts.linux.dev"
 <iommu@...ts.linux.dev>, Thomas Gleixner <tglx@...utronix.de>, Lu Baolu
 <baolu.lu@...ux.intel.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "Hansen, Dave" <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>, "Luse, Paul E" <paul.e.luse@...el.com>, "Williams, Dan
 J" <dan.j.williams@...el.com>, Jens Axboe <axboe@...nel.dk>, "Raj, Ashok"
 <ashok.raj@...el.com>, "maz@...nel.org" <maz@...nel.org>,
 "seanjc@...gle.com" <seanjc@...gle.com>, Robin Murphy
 <robin.murphy@....com>, "jim.harris@...sung.com" <jim.harris@...sung.com>,
 "a.manzanares@...sung.com" <a.manzanares@...sung.com>, Bjorn Helgaas
 <helgaas@...nel.org>, "Zeng, Guang" <guang.zeng@...el.com>,
 "robert.hoo.linux@...il.com" <robert.hoo.linux@...il.com>,
 jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v2 10/13] x86/irq: Extend checks for pending vectors to
 posted interrupts

Hi Kevin,

On Fri, 12 Apr 2024 09:25:57 +0000, "Tian, Kevin" <kevin.tian@...el.com>
wrote:

> > From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > Sent: Saturday, April 6, 2024 6:31 AM
> > 
> > During interrupt affinity change, it is possible to have interrupts
> > delivered to the old CPU after the affinity has changed to the new one.
> > To prevent lost interrupts, local APIC IRR is checked on the old CPU.
> > Similar checks must be done for posted MSIs given the same reason.
> > 
> > Consider the following scenario:
> > 	Device		system agent		iommu
> > 	memory CPU/LAPIC
> > 1	FEEX_XXXX
> > 2			Interrupt request
> > 3						Fetch IRTE	->
> > 4						->Atomic Swap
> > PID.PIR(vec) Push to Global
> > Observable(GO)
> > 5						if (ON*)
> > 	i						done;*  
> 
> there is a stray 'i'
will fix, thanks

> 
> > 						else
> > 6							send a
> > notification ->
> > 
> > * ON: outstanding notification, 1 will suppress new notifications
> > 
> > If the affinity change happens between 3 and 5 in IOMMU, the old CPU's
> > posted
> > interrupt request (PIR) could have pending bit set for the vector being
> > moved.  
> 
> how could affinity change be possible in 3/4 when the cache line is
> locked by IOMMU? Strictly speaking it's about a change after 4 and
> before 6.
SW can still perform affinity change on IRTE and do the flushing on IR
cache after IOMMU fectched it (step 3). They are async events.

In step 4, the atomic swap is on the PID cacheline, not IRTE.

Thanks,

Jacob