Re: [PATCH v2] hw/arm/virt-acpi-build: Fix IORT id_count

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 17, 2024 at 03:39:45PM -0700, Nicolin Chen wrote:
> The IORT doc defines "Number of IDs" ("id_count" in the virt-acpi-build)
> to be "the number of IDs in the range minus one". Otherwise, Linux kernel
> reports "conflicting mapping for input ID" FW_BUG at the overlapped ID.
> 
> Fixes: 42e0f050e3a5 ("hw/arm/virt-acpi-build: Add IORT support to bypass SMMUv3")
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> ---
> Changelog
> v2:
>  * Moved "-1" to the same line of id_count calculation
>  * Added "+1" to the next_range.input_base calculation
> v1:
>  https://lore.kernel.org/all/20240613234802.828265-1-nicolinc@nvidia.com/
> 
>  hw/arm/virt-acpi-build.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
> index c3ccfef026..631f2c6d04 100644
> --- a/hw/arm/virt-acpi-build.c
> +++ b/hw/arm/virt-acpi-build.c
> @@ -243,7 +243,8 @@ iort_host_bridges(Object *obj, void *opaque)
>  
>              AcpiIortIdMapping idmap = {
>                  .input_base = min_bus << 8,
> -                .id_count = (max_bus - min_bus + 1) << 8,
> +                /* id_count is the number of IDs in the range minus one */
> +                .id_count = ((max_bus - min_bus + 1) << 8) - 1,
>              };
>              g_array_append_val(idmap_blob, idmap);
>          }
> @@ -298,11 +299,13 @@ build_iort(GArray *table_data, BIOSLinker *linker, VirtMachineState *vms)
>              idmap = &g_array_index(smmu_idmaps, AcpiIortIdMapping, i);
>  
>              if (next_range.input_base < idmap->input_base) {
> -                next_range.id_count = idmap->input_base - next_range.input_base;
> +                /* id_count is the number of IDs in the range minus one */
> +                next_range.id_count = idmap->input_base -
> +                                      next_range.input_base - 1;
>                  g_array_append_val(its_idmaps, next_range);
>              }
>  
> -            next_range.input_base = idmap->input_base + idmap->id_count;
> +            next_range.input_base = idmap->input_base + idmap->id_count + 1;
>          }


All this has to be written in the way that actually refers to the
spec. id_count is nowhere in the spec and one has to know that
in the end this is used by build_iort_id_mapping to figure out
where this comes from. Not good.

I think the best way is to fix build_iort_id_mapping:
make it subtract 1 from id_count.

Then change text from "Number of IDs" to "Number of IDs - The number of IDs in the range minus one"

You should also add the reference to IO Remapping Table document
near build_iort_id_mapping, it is currently unclear which table this
refers to.

Of couse this means the only correct use has to be tweaked so the change
to build_iort_id_mapping does not break it: 0xFFFF -> 0x10000 - but
that's good for readability, anyway.


>          /* Append the last RC -> ITS ID mapping */
> -- 
> 2.43.0