linux-kernel - Re: [PATCH v2 06/14] iommu: Introduce get_viommu_size and viommu

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Message-ID: <aFCWSZpFuhAweGdb@google.com>
Date: Mon, 16 Jun 2025 22:10:17 +0000
From: Pranjal Shrivastava <praan@...gle.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: jgg@...dia.com, kevin.tian@...el.com, will@...nel.org,
	robin.murphy@....com, joro@...tes.org, yi.l.liu@...el.com,
	peterz@...radead.org, jsnitsel@...hat.com,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
	baolu.lu@...ux.intel.com
Subject: Re: [PATCH v2 06/14] iommu: Introduce get_viommu_size and
 viommu_init ops

On Fri, Jun 13, 2025 at 11:35:18PM -0700, Nicolin Chen wrote:
> So far, a vIOMMU object has been allocated by IOMMU driver and initialized
> with the driver-level structure, before it returns to the iommufd core for
> core-level structure initialization. It has been requiring iommufd core to
> expose some core structure/helpers in its driver.c file, which result in a
> size increase of this driver module.
> 
> Meanwhile, IOMMU drivers are now requiring more vIOMMU-base structures for
> some advanced feature, such as the existing vDEVICE and a future HW_QUEUE.
> Initializing a core-structure later than driver-structure gives for-driver
> helpers some trouble, when they are used by IOMMU driver assuming that the
> new structure (including core) are fully initialized, for example:
> 
> core:	viommu = ops->viommu_alloc();
> driver:	// my_viommu is successfully allocated
> driver:	my_viommu = iommufd_viommu_alloc(...);
> driver:	// This may crash if it reads viommu->ictx
> driver:	new = iommufd_new_viommu_helper(my_viommu->core ...);
> core:	viommu->ictx = ucmd->ictx;
> core:	...
> 
> To ease such a condition, allow the IOMMU driver to report the size of its
> vIOMMU structure, let the core allocate a vIOMMU object and initialize the
> core-level structure first, and then hand it over the driver to initialize
> its driver-level structure.
> 
> Thus, this requires two new iommu ops, get_viommu_size and viommu_init, so
> iommufd core can communicate with drivers to replace the viommu_alloc op:
> 
> core:	viommu = ops->get_viommu_size();
> driver:	return VIOMMU_STRUCT_SIZE();
> core:	viommu->ictx = ucmd->ictx; // and others
> core:	rc = ops->viommu_init();
> driver:	// This is safe now as viommu->ictx is inited
> driver:	new = iommufd_new_viommu_helper(my_viommu->core ...);
> core:	...
> 
> This also adds a VIOMMU_STRUCT_SIZE macro, for drivers to use, which would
> statically sanitize the driver structure.
> 
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>  include/linux/iommu.h   | 15 +++++++++++++++
>  include/linux/iommufd.h |  6 ++++++
>  2 files changed, 21 insertions(+)
> 

Reviewed-by: Pranjal Shrivastava <praan@...gle.com>