linux-kernel - RE: [PATCH v4 11/23] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE

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Message-ID: <BN9PR11MB52761839AABD5AA55DF3FE118C93A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Fri, 16 May 2025 02:49:44 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Nicolin Chen <nicolinc@...dia.com>
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Subject: RE: [PATCH v4 11/23] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE_ALLOC
 ioctl

> From: Nicolin Chen <nicolinc@...dia.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@...dia.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.