linux-kernel - RE: [PATCH v2 08/13] x86/irq: Install posted MSI notification handler

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Date: Thu, 11 Apr 2024 07:52:28 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jacob Pan <jacob.jun.pan@...ux.intel.com>, 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>
CC: "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>
Subject: RE: [PATCH v2 08/13] x86/irq: Install posted MSI notification handler

> From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> Sent: Saturday, April 6, 2024 6:31 AM
> 
> +
> +/*
> + * De-multiplexing posted interrupts is on the performance path, the code
> + * below is written to optimize the cache performance based on the
> following
> + * considerations:
> + * 1.Posted interrupt descriptor (PID) fits in a cache line that is frequently
> + *   accessed by both CPU and IOMMU.
> + * 2.During posted MSI processing, the CPU needs to do 64-bit read and
> xchg
> + *   for checking and clearing posted interrupt request (PIR), a 256 bit field
> + *   within the PID.
> + * 3.On the other side, the IOMMU does atomic swaps of the entire PID
> cache
> + *   line when posting interrupts and setting control bits.
> + * 4.The CPU can access the cache line a magnitude faster than the IOMMU.
> + * 5.Each time the IOMMU does interrupt posting to the PIR will evict the
> PID
> + *   cache line. The cache line states after each operation are as follows:
> + *   CPU		IOMMU			PID Cache line state
> + *   ---------------------------------------------------------------
> + *...read64					exclusive
> + *...lock xchg64				modified
> + *...			post/atomic swap	invalid
> + *...-------------------------------------------------------------
> + *

According to VT-d spec: 5.2.3 Interrupt-Posting Hardware Operation:

"
- Read contents of the Posted Interrupt Descriptor, claiming exclusive
  ownership of its hosting cache-line.
  ...
- Modify the following descriptor field values atomically:
  ...
- Promote the cache-line to be globally observable, so that the modifications
  are visible to other caching agents. Hardware may write-back the cache-line
  anytime after this step.
"

sounds that the PID cache line is not evicted after IOMMU posting?