linux-kernel - Re: [PATCH v1 00/12] iommufd: Repare for IOMMUFD_OBJ_HW

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Message-ID: <aEh6Ma6hLrs78VMA@nvidia.com>
Date: Tue, 10 Jun 2025 11:32:17 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: <jgg@...dia.com>, <kevin.tian@...el.com>
CC: <will@...nel.org>, <robin.murphy@....com>, <joro@...tes.org>,
	<ddutile@...hat.com>, <yi.l.liu@...el.com>, <peterz@...radead.org>,
	<jsnitsel@...hat.com>, <praan@...gle.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 v1 00/12] iommufd: Repare for IOMMUFD_OBJ_HW_QUEUE

On Mon, Jun 09, 2025 at 10:13:23AM -0700, Nicolin Chen wrote:
> The new HW Queue object will require more interactions with IOMMU drivers,
> with a few more for-driver APIs. This will complicate the driver-allocated
> structure design like the viommu_alloc op: since the core structure is not
> initialized during the driver allocation stage, a new for-driver API can't
> reference any member in the core vIOMMU structure.
> 
> Make a preparatory series doing:

So, Don pointed out that "Repare" is a typo here. All this series
doing is to "Prepare", as IOMMUFD_OBJ_HW_QUEUE hasn't been merged
yet.

Also the main goal of this series is to replace viommu_alloc with
get_viommu_size + viommu_init. The patch that introduces the two
new ops has the full details (though I should have some narrative
clearly show the idea in the cover letter too):

--------------------------------------------------------------------------
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:
    // my_viommu is successfully allocated
    my_viommu = iommufd_viommu_alloc(...);
    // This may crash if it reads viommu->ictx
    new = iommufd_new_viommu_helper(my_viommu->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.

This also adds a VIOMMU_STRUCT_SIZE macro, for drivers to use, which would
statically sanitize the driver structure.
--------------------------------------------------------------------------

Thanks.