linux-kernel - Re: Virtio-IOMMU interrupt remapping design

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Message-ID: <20250616132031.GB1354058@ziepe.ca>
Date: Mon, 16 Jun 2025 10:20:31 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Demi Marie Obenour <demiobenour@...il.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Len Brown <lenb@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Alyssa Ross <hi@...ssa.is>, virtualization@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	iommu@...ts.linux.dev, x86@...nel.org,
	Spectrum OS Development <devel@...ctrum-os.org>
Subject: Re: Virtio-IOMMU interrupt remapping design

On Sun, Jun 15, 2025 at 02:47:15PM -0400, Demi Marie Obenour wrote:

> Is a paravirtualized IOMMU with interrupt remapping something that makes
> sense?

IMHO linking interrupt remapping to the iommu is a poor design,
interrupt routing belongs in the irq subsystem, not in the iommu.

The fact AMD and Intel both coupled their interrupt routing to their
iommu hardware is just a weird design decision. ARM didn't do this,
for instance.

So I would not try to do this at all, you should have a
para-virtualized IRQ interface, not an extension to virtio-iommu
adding interrupt handling. :\

AFAIK hyperv shows how to build something like this.

Jason