linux-kernel - Re: [PATCH v4 1/3] iommu: Sort out domain user data

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Message-ID: <20250317193742.GV9311@nvidia.com>
Date: Mon, 17 Mar 2025 16:37:42 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, robin.murphy@....com, joro@...tes.org,
	will@...nel.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] iommu: Sort out domain user data

On Thu, Mar 06, 2025 at 01:00:47PM -0800, Nicolin Chen wrote:
> From: Robin Murphy <robin.murphy@....com>
> 
> When DMA/MSI cookies were made first-class citizens back in commit
> 46983fcd67ac ("iommu: Pull IOVA cookie management into the core"), there
> was no real need to further expose the two different cookie types.
> However, now that IOMMUFD wants to add a third type of MSI-mapping
> cookie, we do have a nicely compelling reason to properly dismabiguate
> things at the domain level beyond just vaguely guessing from the domain
> type.
> 
> Meanwhile, we also effectively have another "cookie" in the form of the
> anonymous union for other user data, which isn't much better in terms of
> being vague and unenforced. The fact is that all these cookie types are
> mutually exclusive, in the sense that combining them makes zero sense
> and/or would be catastrophic (iommu_set_fault_handler() on an SVA
> domain, anyone?) - the only combination which *might* be reasonable is
> perhaps a fault handler and an MSI cookie, but nobody's doing that at
> the moment, so let's rule it out as well for the sake of being clear and
> robust. To that end, we pull DMA and MSI cookies apart a little more,
> mostly to clear up the ambiguity at domain teardown, then for clarity
> (and to save a little space), move them into the union, whose ownership
> we can then properly describe and enforce entirely unambiguously.
> 
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> Reviewed-by: Kevin Tian <kevin.tian@...el.com>
> [nicolinc: rebase on latest tree; use prefix IOMMU_COOKIE_; merge unions
>            in iommu_domain; add IOMMU_COOKIE_IOMMUFD for iommufd_hwpt]
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>  drivers/iommu/dma-iommu.h            |   5 +
>  include/linux/iommu.h                |  20 ++-
>  drivers/iommu/dma-iommu.c            | 194 ++++++++++++++-------------
>  drivers/iommu/iommu-sva.c            |   1 +
>  drivers/iommu/iommu.c                |  18 ++-
>  drivers/iommu/iommufd/hw_pagetable.c |   3 +
>  6 files changed, 143 insertions(+), 98 deletions(-)

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

Jason