linux-kernel - Re: [PATCH v4 10/23] iommufd/viommu: Introduce IOMMUFD_OBJ_HW

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Message-ID: <aCajRTEU3vPQAzpD@Asurada-Nvidia>
Date: Thu, 15 May 2025 19:30:29 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
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Subject: Re: [PATCH v4 10/23] iommufd/viommu: Introduce IOMMUFD_OBJ_HW_QUEUE
 and its related struct

On Thu, May 15, 2025 at 05:58:41AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@...dia.com>
> > Sent: Friday, May 9, 2025 11:03 AM
> > 
> > Add IOMMUFD_OBJ_HW_QUEUE with an iommufd_hw_queue structure,
> > representing
> > a HW-accelerated queue type of IOMMU's physical queue that can be passed
> > through to a user space VM for direct hardware control, such as:
> >  - NVIDIA's Virtual Command Queue
> >  - AMD vIOMMU's Command Buffer, Event Log Buffer, and PPR Log Buffer
> > 
> > Introduce an allocator iommufd_hw_queue_alloc(). And add a pair of
> > viommu
> > ops for iommufd to forward user space ioctls to IOMMU drivers.
> > 
> > Given that the first user of this HW QUEUE (tegra241-cmdqv) will need to
> > ensure the queue memory to be physically contiguous, add a flag property
> > in iommufd_viommu_ops and
> > IOMMUFD_VIOMMU_FLAG_HW_QUEUE_READS_PA to allow
> > driver to flag it so that the core will validate the physical pages of a
> > given guest queue.
> 
> 'READS' is confusing here. What about xxx_CONTIG_PAS?

Combining Jason's first comments here:
https://lore.kernel.org/linux-iommu/20250515160620.GJ382960@nvidia.com/

So, pinning should be optional too. And I think there would be
unlikely a case where HW needs contiguous physical pages while
not requiring to pin the pages, right?

So, we need an flag that could indicate to do both tests. Yet,
"xxx_CONTIG_PAS" doesn't sound very fitting, compared to this
"IOMMUFD_VIOMMU_FLAG_HW_QUEUE_READS_PA".

Perhaps, we should just add some comments to clarify a bit. Or
do you have some better naming?

Thanks
Nicolin