Re: Virtio-IOMMU interrupt remapping design

["Michael S. Tsirkin" <mst@redhat.com>]

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.