Re: [PATCH v3 4/4] x86/ucode: Add Kconfig option to remove microcode loading

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.01.2026 13:21, Alejandro Vallejo wrote:
> @@ -469,7 +471,7 @@ struct ucode_buf {
>      char buffer[];
>  };
>  
> -static long cf_check ucode_update_hcall_cont(void *data)
> +static long cf_check __maybe_unused ucode_update_hcall_cont(void *data)
>  {
>      struct microcode_patch *patch = NULL;
>      int ret, result;

Why this change when ...

> @@ -613,6 +615,7 @@ static long cf_check ucode_update_hcall_cont(void *data)
>      return ret;
>  }
>  
> +#ifdef CONFIG_MICROCODE_LOADING

... this can simply be moved up accordingly? After all ...

>  int ucode_update_hcall(XEN_GUEST_HANDLE(const_void) buf,
>                         unsigned long len, unsigned int flags)
>  {
> @@ -645,6 +648,7 @@ int ucode_update_hcall(XEN_GUEST_HANDLE(const_void) buf,
>       */
>      return continue_hypercall_on_cpu(0, ucode_update_hcall_cont, buffer);

... this is the only user of that other function.

> --- a/xen/arch/x86/cpu/microcode/intel.c
> +++ b/xen/arch/x86/cpu/microcode/intel.c
> @@ -408,17 +408,20 @@ static const char __initconst intel_cpio_path[] =
>      "kernel/x86/microcode/GenuineIntel.bin";
>  
>  static const struct microcode_ops __initconst_cf_clobber intel_ucode_ops = {
> -    .cpu_request_microcode            = cpu_request_microcode,
> +    .cpu_request_microcode            = MICROCODE_OP(cpu_request_microcode),
>      .collect_cpu_info                 = collect_cpu_info,
> -    .apply_microcode                  = apply_microcode,
> -    .compare                          = intel_compare,
> -    .cpio_path                        = intel_cpio_path,
> +    .apply_microcode                  = MICROCODE_OP(apply_microcode),
> +    .compare                          = MICROCODE_OP(intel_compare),
> +    .cpio_path                        = MICROCODE_OP(intel_cpio_path),
>  };

While I appreciate the intention with MICROCODE_OP(), I'm not really happy
with function pointer members left in place just for them to be NULL
everywhere. What if a call site remains unguarded? With PV guests that
would be a privilege escalation XSA.

Jan