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Re: [PATCH] xen/amd/iommu: record the IOMMU for alias DeviceIDs
- To: Stefano Stabellini <stefano.stabellini@xxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 26 Oct 2026 14:25:29 +0100
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- Cc: andrew.cooper3@xxxxxxxxxx, roger@xxxxxxxxxxxxxx, jason.andryuk@xxxxxxx, teddy.astie@xxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 26 Oct 2026 13:25:40 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 21.10.2026 00:12, Stefano Stabellini wrote:
> add_ivrs_mapping_entry() marks an alias DeviceID valid and may give it
> an interrupt remapping table, but records the IOMMU only for the
> aliased device. If no IVHD entry names the alias DeviceID itself,
> amd_iommu_setup_device_table() passes a NULL IOMMU to
> amd_iommu_set_intremap_table(), and Xen crashes at boot.
>
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
The description deals with only part of what's needed for justification.
What's missing is analysis of the safety of the change being made: There
may very well be logic somewhere which relies on alias entries to be
NULL. If there were (which I seem to have a faint recollection of), this
change wouldn't be safe to make, and checks may instead be necessary to
add elsewhere to circumvent the (vaguely described) boot time crash(es).
Jan
> --- a/xen/drivers/passthrough/amd/iommu_acpi.c
> +++ b/xen/drivers/passthrough/amd/iommu_acpi.c
> @@ -104,6 +104,10 @@ static void __init add_ivrs_mapping_entry(
>
> /* Assign IOMMU hardware. */
> ivrs_mappings[bdf].iommu = iommu;
> +
> + /* Requests using the alias go through the same IOMMU. */
> + if ( !ivrs_mappings[alias_id].iommu )
> + ivrs_mappings[alias_id].iommu = iommu;
> }
>
> static struct amd_iommu * __init find_iommu_from_bdf_cap(
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