linux-kernel - Re: [PATCH v2 11/22] iommufd: Add for-driver helpers iommufd_vcmdq

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Message-ID: <bfb7dd4a-0597-4b6c-ba53-1e4ee3b733b0@linux.intel.com>
Date: Mon, 28 Apr 2025 10:22:09 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Nicolin Chen <nicolinc@...dia.com>, jgg@...dia.com, kevin.tian@...el.com,
 corbet@....net, will@...nel.org
Cc: 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
Subject: Re: [PATCH v2 11/22] iommufd: Add for-driver helpers
 iommufd_vcmdq_depend/undepend()

On 4/26/25 13:58, 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
> the destroy op returns void.

The key point is that unmapping happens during object destroy. These
depend/undepend helpers ensure a vCMDQ is not destroyed (and therefore
unmapped) before any vCMDQs that depend on it. Do I get it right?

> 
> Add iommufd_vcmdq_depend/undepend() for-driver helpers, allowing LVCMDQ
> allocator to refcount_inc() a sibling LVCMDQ object and LVCMDQ destroyer
> to refcount_dec().
> 
> 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.
> 
> Signed-off-by: Nicolin Chen<nicolinc@...dia.com>

... if that's right,

Reviewed-by: Lu Baolu <baolu.lu@...ux.intel.com>

Thanks,
baolu