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

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Message-ID: <20250617154331-mutt-send-email-mst@kernel.org>
Date: Tue, 17 Jun 2025 15:44:20 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Demi Marie Obenour <demiobenour@...il.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 Mon, Jun 16, 2025 at 10:20:31AM -0300, Jason Gunthorpe wrote:
> 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.


why does it matter in which device it resides?
Way I see it, there is little reason to remap interrupts
without also using an iommu, so why not a single device.
what did I miss?


> 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