Re: Virtio-IOMMU interrupt remapping design

Demi Marie Obenour
2025-06-16T16:54:02Z

[-- Attachment #1.1.1: Type: text/plain, Size: 984 bytes --]

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)

[-- Attachment #1.1.2: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 7253 bytes --]

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]