linux-kernel - Re: [PATCH v6 11/25] iommufd/driver: Add iommufd_hw_queue

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Message-ID: <20250616140608.GF1174925@nvidia.com>
Date: Mon, 16 Jun 2025 11:06:08 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, corbet@....net, will@...nel.org,
	bagasdotme@...il.com, robin.murphy@....com, joro@...tes.org,
	thierry.reding@...il.com, vdumpa@...dia.com, jonathanh@...dia.com,
	shuah@...nel.org, jsnitsel@...hat.com, nathan@...nel.org,
	peterz@...radead.org, yi.l.liu@...el.com, mshavit@...gle.com,
	praan@...gle.com, zhangzekun11@...wei.com, iommu@...ts.linux.dev,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-tegra@...r.kernel.org,
	linux-kselftest@...r.kernel.org, patches@...ts.linux.dev,
	mochs@...dia.com, alok.a.tiwari@...cle.com, vasant.hegde@....com,
	dwmw2@...radead.org, baolu.lu@...ux.intel.com
Subject: Re: [PATCH v6 11/25] iommufd/driver: Add
 iommufd_hw_queue_depend/undepend() helpers

On Sat, Jun 14, 2025 at 12:14:36AM -0700, Nicolin Chen wrote:
> NVIDIA Virtual Command Queue is one of the iommufd users exposing vIOMMU
> features to user space VMs. Its hardware has a strict rule when mapping
> and unmapping multiple global CMDQVs to/from a VM-owned VINTF, requiring
> mappings in ascending order and unmappings in descending order.
> 
> The tegra241-cmdqv driver can apply the rule for a mapping in the LVCMDQ
> allocation handler. However, it can't do the same for an unmapping since
> user space could start random destroy calls breaking the rule, while the
> destroy op in the driver level can't reject a destroy call as it returns
> void.
> 
> Add iommufd_hw_queue_depend/undepend for-driver helpers, allowing LVCMDQ
> allocator to refcount_inc() a sibling LVCMDQ object and LVCMDQ destroyer
> to refcount_dec(), so that iommufd core will help block a random destroy
> call that breaks the rule.
> 
> This is a bit of compromise, because a driver might end up with abusing
> the API that deadlocks the objects. So restrict the API to a dependency
> between two driver-allocated objects of the same type, as iommufd would
> unlikely build any core-level dependency in this case. And encourage to
> use the macro version that currently supports the HW QUEUE objects only.
> 
> Reviewed-by: Lu Baolu <baolu.lu@...ux.intel.com>
> Reviewed-by: Pranjal Shrivastava <praan@...gle.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>  include/linux/iommufd.h        | 44 ++++++++++++++++++++++++++++++++++
>  drivers/iommu/iommufd/driver.c | 28 ++++++++++++++++++++++
>  2 files changed, 72 insertions(+)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason