Re: [PATCH v4 11/23] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE_ALLOC ioctl

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, May 16, 2025 at 02:49:44AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@nvidia.com>
> > Sent: Friday, May 16, 2025 2:45 AM
> > 
> > On Thu, May 15, 2025 at 06:30:27AM +0000, Tian, Kevin wrote:
> > > > From: Nicolin Chen <nicolinc@nvidia.com>
> > > > Sent: Friday, May 9, 2025 11:03 AM
> > > >
> > > > +
> > > > +/**
> > > > + * struct iommu_hw_queue_alloc - ioctl(IOMMU_HW_QUEUE_ALLOC)
> > > > + * @size: sizeof(struct iommu_hw_queue_alloc)
> > > > + * @flags: Must be 0
> > > > + * @viommu_id: Virtual IOMMU ID to associate the HW queue with
> > > > + * @type: One of enum iommu_hw_queue_type
> > > > + * @index: The logical index to the HW queue per virtual IOMMU for a
> > > > multi-queue
> > > > + *         model
> > >
> > > I'm thinking of an alternative way w/o having the user to assign index
> > > and allowing the driver to poke object dependency (next patch).
> > >
> > > Let's say the index is internally assigned by the driver. so this cmd is
> > > just for allowing a hw queue and it's the driver to decide the allocation
> > > policy, e.g. in ascending order.
> > >
> > > Introduce a new flag in viommu_ops to indicate to core that the
> > > new hw queue should hold a reference to the previous hw queue.
> > >
> > > core maintains a last_queue field in viommu. Upon success return
> > > from @hw_queue_alloc() the core increments the users refcnt of
> > > last_queue, records the dependency in iommufd_hw_queue struct,
> > > and update viommu->last_queue.
> > >
> > > Then the destroy order is naturally guaranteed.
> > 
> > I have thought about that too. It's nice that the core can easily
> > maintain the dependency for the driver.
> > 
> > But there would still need an out_index to mark each dynamically
> > allocated queue. So VMM would know where it should map the queue.
> > 
> > For example, if VMM wants to allocate a queue at its own index=1
> > without allocating index=0 first, kernel cannot fail that as VMM
> > doesn't provide the index. The only way left for kernel would be
> > to output the allocated queue with index=0 and then wish VMM can
> > validate it, which doesn't sound safe..
> > 
> 
> VMM's index is virtual which could be mapped to whatever queue
> object created at its own disposal.
> 
> the uAPI just requires VMM to remember a sequential list of  allocated
> queue objects and destroy them in reverse order of allocation, instead
> of in the reverse order of virtual indexes.

But that's not going to work for VCMDQ.

VINTF mmaps only a single page that controls multiple queues. And
all queues have to be mapped correctly between HW and VM indexes.
Otherwise, it won't work if VMM maps:

HW-level VINTF1 LVCMDQ0 <==> VM-level VINTF0 LVCMDQ1
HW-level VINTF1 LVCMDQ1 <==> VM-level VINTF0 LVCMDQ0

So, one way or another, kernel has to ensure the static mappings
of the indexes. And I think it's safer in the way that VMM tells
what index to allocate..

Thanks
Nicolin