Re: [PATCH v2] xen/arm: fix arm_iommu_map_page after f9f6b22ab

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

On 7/10/25 20:25, Stefano Stabellini wrote:
> Up until f9f6b22ab "xen/arm: Map ITS doorbell register to IOMMU page
> tables" the only caller of iommu_map on ARM was grant_table.c which has
> a specific usage model and restrictions as described by the in-code
> comment in arm_iommu_map_page.
> 
> f9f6b22ab introduced a second caller to iommu_map on ARM:
> vgic_v3_its_init_virtual. This specific statement in the
> f9f6b22ab commit message is wrong:
> 
> "Note that the 1:1 check in arm_iommu_map_page remains for now, as
> virtual ITSes are currently only created for hwdom where the doorbell
> mapping is always 1:1."
> 
> Leading to crashes any time the hardware domain is not direct-mapped
> (e.g. cache coloring and non-Dom0 hardware domain):
> 
> (XEN) Xen BUG at drivers/passthrough/arm/iommu_helpers.c:49
> [...]
> (XEN) Xen call trace:
> (XEN)    [<00000a000024c758>] arm_iommu_map_page+0x80/0x90 (PC)
> (XEN)    [<00000a000024c750>] arm_iommu_map_page+0x78/0x90 (LR)
> (XEN)    [<00000a0000250884>] iommu_map+0xcc/0x29c
> (XEN)    [<00000a0000288024>] vgic_v3_its_init_domain+0x18c/0x1e8
> (XEN)    [<00000a0000285228>] vgic-v3.c#vgic_v3_domain_init+0x168/0x21c
> (XEN)    [<00000a0000281dcc>] domain_vgic_init+0x14c/0x210
> (XEN)    [<00000a00002705a4>] arch_domain_create+0x150/0x1f0
> (XEN)    [<00000a00002055e8>] domain_create+0x47c/0x6c0
> (XEN)    [<00000a00002cf090>] create_domUs+0x7f8/0x8cc
> (XEN)    [<00000a00002eb588>] start_xen+0x8f4/0x998
> (XEN)    [<00000a000020018c>] head.o#primary_switched+0x4/0x10
> 
> Specifically, non-1:1 hardware domain exists with cache coloring
> enabled. For that, is_domain_direct_mapped(d) is false but
> domain_use_host_layout(d) is true.
> 
> At a minimum, we need to change the is_domain_direct_mapped(d) check in
> arm_iommu_map_page into a domain_use_host_layout(d) check. But in
> preparation of exposing vITS to domUs, let's take this opportunity to
> generalize the arm_iommu_map_page function further to be more generic
> and unopinionated. Move the in-code comment specific to the grant table
> can live in grant-table.c instead.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>

Reviewed-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>

> ---
>  xen/common/grant_table.c                    | 10 ++++++++--
>  xen/drivers/passthrough/arm/iommu_helpers.c | 13 +------------
>  2 files changed, 9 insertions(+), 14 deletions(-)
> 
> diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
> index cf131c43a1..2e08dac656 100644
> --- 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 IPA). 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.
> +         *
> +         * 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;
> diff --git a/xen/drivers/passthrough/arm/iommu_helpers.c 
> b/xen/drivers/passthrough/arm/iommu_helpers.c
> index 5cb1987481..dae5fc0caa 100644
> --- a/xen/drivers/passthrough/arm/iommu_helpers.c
> +++ b/xen/drivers/passthrough/arm/iommu_helpers.c
> @@ -36,17 +36,6 @@ int __must_check arm_iommu_map_page(struct domain *d, 
> dfn_t dfn, mfn_t mfn,
>  {
>      p2m_type_t t;
>  
> -    /*
> -     * Grant mappings can be used for DMA requests. The dev_bus_addr
> -     * returned by the hypercall is the MFN (not the IPA). 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. Hence this
> -     * function should only be used by gnttab code with gfn == mfn == dfn.
> -     */
> -    BUG_ON(!is_domain_direct_mapped(d));
> -    BUG_ON(mfn_x(mfn) != dfn_x(dfn));
> -
>      /* We only support readable and writable flags */
>      if ( !(flags & (IOMMUF_readable | IOMMUF_writable)) )
>          return -EINVAL;
> @@ -57,7 +46,7 @@ int __must_check arm_iommu_map_page(struct domain *d, dfn_t 
> dfn, mfn_t mfn,
>       * The function guest_physmap_add_entry replaces the current mapping
>       * if there is already one...
>       */
> -    return guest_physmap_add_entry(d, _gfn(dfn_x(dfn)), _mfn(dfn_x(dfn)),
> +    return guest_physmap_add_entry(d, _gfn(dfn_x(dfn)), mfn,
>                                     IOMMUF_order(flags), t);
>  }
>