Re: [PATCH v2] acpi/arm: relax MADT GICC entry length check to support newer ACPI revisions

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.01.2026 17:34, Oleksii Kurochko wrote:
> Newer ACPI revisions define the MADT GICC entry with Length = 82 bytes [1].
> The current BAD_MADT_GICC_ENTRY() check rejects entries whose length does not
> match the known values, which leads to:
>   GICv3: No valid GICC entries exist.
> as observed on the AmpereOne platform.
> 
> To fix this, import the logic from Linux commit 9eb1c92b47c7:
>   The BAD_MADT_GICC_ENTRY check is a little too strict because
>   it rejects MADT entries that don't match the currently known
>   lengths. We should remove this restriction to avoid problems
>   if the table length changes. Future code which might depend on
>   additional fields should be written to validate those fields
>   before using them, rather than trying to globally check
>   known MADT version lengths.
> 
>   Link: 
> https://lkml.kernel.org/r/20181012192937.3819951-1-jeremy.linton@xxxxxxx
>   Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
>   [lorenzo.pieralisi@xxxxxxx: added MADT macro comments]
>   Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx>
>   Acked-by: Sudeep Holla <sudeep.holla@xxxxxxx>
>   Cc: Will Deacon <will.deacon@xxxxxxx>
>   Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>   Cc: Al Stone <ahs3@xxxxxxxxxx>
>   Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
>   Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> 
> As ACPI_MADT_GICC_LENGTH is dropped, update the functions where it is
> used. As we rewrite the MADT for hwdom, reuse the host GICC header length
> instead of ACPI_MADT_GICC_LENGTH.
> 
> [1] 
> https://uefi.org/specs/ACPI/6.6/05_ACPI_Software_Programming_Model.html#gic-cpu-interface-gicc-structure
> 
> Reported-By: Yann Dirson <yann.dirson@xxxxxxxxxx>
> Co-developed-by: Yann Sionneau <yann.sionneau@xxxxxxxxxx>
> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> ---
> I ran CI tests where it made sense for this patch, as I don’t see any CI job
> that builds Xen with CONFIG_ACPI=y:
>   https://gitlab.com/xen-project/people/olkur/xen/-/pipelines/2229673951
> 
> I also built Xen manually with CONFIG_ACPI=y enabled and tested it on the
> AmpereOne platform.
> ---
> Changes in v2:
>  - Update the commit message:
>    - Use more characters for commit ID.
>    - Drop 'import from'.
>  - Add Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>.

Was this a legitimate thing to do, considering ...

>  - Make the local variables host_gicc const in  gic_get_hwdom_madt_size().
>    (header variable isn't const as container_of() will discard 'const' 
> qualifier
>    and so compilation error will occur).
>  - Return 0 instead of panic() in gic_get_hwdom_madt_size().

... all of these (plus leaving partly unaddressed a comment from Julien)?

> --- a/xen/arch/arm/gic.c
> +++ b/xen/arch/arm/gic.c
> @@ -418,8 +418,18 @@ unsigned long gic_get_hwdom_madt_size(const struct 
> domain *d)
>  {
>      unsigned long madt_size;
>  
> +    struct acpi_subtable_header *header;

Why is there a blank line left between declarations? In unusual situations (very
many variables, for example) that may be okay, but otherwise the first blank 
line
generally wants to separate decls from statements.

Also Julien asked for this to be const. You claimed a compile error would occur
if you do, but afaict that's only because ...

> +    const struct acpi_madt_generic_interrupt *host_gicc;
> +
> +    header = acpi_table_get_entry_madt(ACPI_MADT_TYPE_GENERIC_INTERRUPT, 0);
> +    if ( !header )
> +        return 0;
> +
> +    host_gicc = container_of(header, struct acpi_madt_generic_interrupt,
> +                             header);

... you don't use const properly here as well.

Finally (possibly not for this patch, but mentioning since originally it was
pointed out as an option) the function imo wants to become __init anyway, for
(as said by Julien) its only caller being so.

Jan