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

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Message-ID: <20250617154524-mutt-send-email-mst@kernel.org>
Date: Tue, 17 Jun 2025 15:46:12 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Demi Marie Obenour <demiobenour@...il.com>
Cc: 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:
> Virtio-IOMMU interrupt remapping turned out to be much harder than I
> realized.  The main problem is that interrupt remapping is set up
> very early in boot.  In fact, Linux calls the interrupt remapping probe
> function from the APIC initialization code: x86_64_probe_apic ->
> enable_IR_x2apic -> irq_remapping_prepare().  This is almost certainly
> much before PCI has been initialized.  Also, the order in which devices
> will be initialized is not something Linux guarantees at all, which is a
> problem because interrupt remapping must be initialized before drivers
> start setting up interrupts.  Otherwise, the interrupt remapping table
> won't include entries for already-existing interrupts, and things will
> either break badly, not get the benefit of interrupt remapping
> security-wise, or both.
> 
> The reason I expect this doesn't cause problems for address translation
> is that the IOMMU probably starts in bypass mode by default, meaning
> that all DMA is permitted.  If the IOMMU is only used by VFIO or
> IOMMUFD, it will not be needed until userspace starts up, which is after
> the IOMMU has been initialized.  This isn't ideal, though, as it means
> that kernel drivers operate without DMA protection.
> 
> Is a paravirtualized IOMMU with interrupt remapping something that makes
> sense?  Absolutely!  However, the IOMMU should be considered a platform
> device that must be initialized very early in boot.  Using virtio-IOMMU
> with MMIO transport as the interface might be a reasonable option, but
> the IOMMU needs to be enumerated via ACPI, device tree, or kernel
> command line argument.  This allows it to be brought up before anything
> capable of DMA is initialized.
> 
> Is this the right path to go down?  What do others think about this?
> -- 
> Sincerely,
> Demi Marie Obenour (she/her/hers)



The project for this discussion is also virtio-comment,
this ML is for driver work.