linux-kernel - Re: [PATCH] iommu: Sort out domain user data

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Message-ID: <20250228173857.GD39591@nvidia.com>
Date: Fri, 28 Feb 2025 13:38:57 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Robin Murphy <robin.murphy@....com>
Cc: will@...nel.org, joro@...tes.org, iommu@...ts.linux.dev,
	nicolinc@...dia.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu: Sort out domain user data

On Fri, Feb 28, 2025 at 11:31:11AM +0000, Robin Murphy wrote:
> 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>
> ---
>  drivers/iommu/dma-iommu.c | 189 ++++++++++++++++++++------------------
>  drivers/iommu/dma-iommu.h |   5 +
>  drivers/iommu/iommu-sva.c |   1 +
>  drivers/iommu/iommu.c     |  17 +++-
>  include/linux/iommu.h     |  13 ++-
>  5 files changed, 134 insertions(+), 91 deletions(-)

This looks good to me, especially the cleanup of the fault stuff. Do
you want Nicolin to rebase it and combine it with the other bits he
has?

Jason