Re: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable nested SMMUv3

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Feb 06, 2025 at 12:33:19PM -0800, Nicolin Chen wrote:
> On Thu, Feb 06, 2025 at 02:22:01PM -0400, Jason Gunthorpe wrote:
> > On Thu, Feb 06, 2025 at 06:18:14PM +0000, Shameerali Kolothum Thodi wrote:
> > 
> > > > So even if you invent an iommu ID we cannot accept it as a handle to
> > > > create viommu in iommufd.
> > > 
> > > Creating the vIOMMU only happens when the user does a  cold/hot plug of
> > > a VFIO device. At that time Qemu checks whether the assigned id matches
> > > with whatever the kernel tell it. 
> > 
> > This is not hard up until the guest is started. If you boot a guest
> > without a backing viommu iommufd object then there will be some more
> > complexities.
> 
> Yea, I imagined that things would be complicated with hotplugs..
> 
> On one hand, I got the part that we need some fixed link forehand
> to ease migration/hotplugs.
> 
> On the other hand, all IOMMUFD ioctls need a VFIO device FD, which
> brings the immediate attention that we cannot even decide vSMMU's
> capabilities being reflected in its IDR/IIDR registers, without a
> coldplug device

As Daniel was saying this all has to be specifiable on the command
line.

IMHO if the vSMMU is not fully specified by the time the boot happens
(either explicity via command line or implicitly by querying the live
HW) then it qemu should fail.

Jason