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

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Message-ID: <8c3b08d5-24aa-4db2-84e1-dfd1d2c52065@gmail.com>
Date: Mon, 16 Jun 2025 12:53:40 -0400
From: Demi Marie Obenour <demiobenour@...il.com>
To: Jason Gunthorpe <jgg@...pe.ca>
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 6/16/25 09:20, 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.

I agree.

> 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.

Arm did the right thing here, IMO.

> 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. :\

I don't disagree at all.

> AFAIK hyperv shows how to build something like this.
Would this need KVM patches?  I'm concerned that implementing this
in userspace would interact badly with the irqfd fast path.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
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