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 12:48:40PM -0800, Nicolin Chen wrote:
> On Thu, Feb 06, 2025 at 04:38:55PM -0400, Jason Gunthorpe wrote:
> > 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.
> 
> Though that makes sense, that would assume we could only support
> the case where a VM has at least one cold plug device per vSMMU?
> 
> Otherwise, even if we specify vSMMU to which pSMMU via a command
> line, we can't get access to the pSMMU via IOMMU_GET_HW_INFO..

You'd use the command line information and wouldn't need GET_HW_INFO,
it would be complicated

Jason