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

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

On Thu, Jun 13, 2024 at 04:48:02PM -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>
> ---
>  hw/arm/virt-acpi-build.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
> index c3ccfef026..b9343dde0f 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);
>          }
> @@ -298,7 +299,9 @@ 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) {
> +                /* id_count is the number of IDs in the range minus one */
>                  next_range.id_count = idmap->input_base - next_range.input_base;
> +                next_range.id_count -= 1;

I would just add - 1 on the previous line, instead of making it
incorrect then correcting it.

>                  g_array_append_val(its_idmaps, next_range);
>              }


But the value is used later:

            next_range.input_base = idmap->input_base + idmap->id_count;

Wouldn't that make next_range incorrect?


I also note that

static void build_iort_id_mapping(GArray *table_data, uint32_t input_base,
                                  uint32_t id_count, uint32_t out_ref)
{
    /* Table 4 ID mapping format */
    build_append_int_noprefix(table_data, input_base, 4); /* Input base */
    build_append_int_noprefix(table_data, id_count, 4); /* Number of IDs */
    build_append_int_noprefix(table_data, input_base, 4); /* Output base */
    build_append_int_noprefix(table_data, out_ref, 4); /* Output Reference */
    /* Flags */
    build_append_int_noprefix(table_data, 0 /* Single mapping (disabled) */, 4);
}


That comment 
    /* Table 4 ID mapping format */

really should be before the function and it should mention the spec
it's from - specifically the earliest spec including the relevant table.





>  
> -- 
> 2.43.0