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

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Message-ID: <20250616173305.GA1376249@ziepe.ca>
Date: Mon, 16 Jun 2025 14:33:05 -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 Mon, Jun 16, 2025 at 12:53:40PM -0400, Demi Marie Obenour wrote:

> > 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.

I don't know. I think you get the same issues even if you did
virtio-iommu irq handling, it shouldn't be any different.

I'm not sure there even is a fast path here, remapping happens during
initial vector setup/affinity change only. That isn't fast path. So
long as the MSI is delivered to the correct CPU vector entirely in KVM
it seems OK.

And the hyperv approach of asking the hypervisor for the addr/data
pair to achieve certain parameters will work alot better with existing
Linux than trying to build a iommu emulation where the guest is
building its own private addr/data pairs :\

Jason