Re: [PATCH v3 1/2] xen/arm: fix arm_iommu_map_page after f9f6b22abf1d

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.07.2025 00:51, Stewart Hildebrand wrote:
> --- a/xen/common/grant_table.c
> +++ b/xen/common/grant_table.c
> @@ -1274,8 +1274,14 @@ map_grant_ref(
>          }
>  
>          /*
> -         * We're not translated, so we know that dfns and mfns are
> -         * the same things, so the IOMMU entry is always 1-to-1.
> +         * Grant mappings can be used for DMA requests. The dev_bus_addr
> +         * returned by the hypercall is the MFN (not the GFN). For
> +         * device protected by an IOMMU, Xen needs to add a 1:1 mapping
> +         * in the domain p2m to allow DMA request to work. This is only
> +         * valid when the domain is directed mapped.

While true for Arm, the concept of "direct mapped" doesn't really exist on
x86 (and only the PV case could make it here, which is sufficiently
different anyway). If you really think the comment here needs adjustment,
please make sure it remains correct for all architectures.

Jan

> +         * We're not translated, so we know that dfns and mfns are the
> +         * same things, so the IOMMU entry is always 1-to-1.
>           */
>          if ( !(op->flags & GNTMAP_readonly) && node.cnt.wr == 1 )
>              kind = IOMMUF_readable | IOMMUF_writable;