linux-kernel - RE: [PATCH v3 1/3] iommu: Sort out domain user data

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Message-ID: <BN9PR11MB527602ABF245960E85402C588CCA2@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Thu, 6 Mar 2025 05:59:03 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Nicolin Chen <nicolinc@...dia.com>, "jgg@...dia.com" <jgg@...dia.com>,
	"robin.murphy@....com" <robin.murphy@....com>, "joro@...tes.org"
	<joro@...tes.org>, "will@...nel.org" <will@...nel.org>
CC: "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3 1/3] iommu: Sort out domain user data

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Tuesday, March 4, 2025 4:53 AM
> 
> 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>
> [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>

Reviewed-by: Kevin Tian <kevin.tian@...el.com>

with a nit:

>  	msi_addr &= ~(phys_addr_t)(size - 1);
> -	list_for_each_entry(msi_page, &cookie->msi_page_list, list)
> +	list_for_each_entry(msi_page, cookie_msi_pages(domain), list)
>  		if (msi_page->phys == msi_addr)
>  			return msi_page;
> 

Above checks cookie type in every iteration. Better save the list
pointer to a local variable.