efi: Allow to enable EFI runtime services by default on RT

Commit d9f283ae ("efi: Disable runtime services on RT") disabled EFI
runtime services by default when the CONFIG_PREEMPT_RT option is enabled.

The rationale for that commit is that some EFI calls could take too much
time, leading to large latencies which is an issue for Real-Time kernels.

But a side effect of that change was that now is not possible anymore to
enable the EFI runtime services by default when CONFIG_PREEMPT_RT is set,
without passing an efi=runtime command line parameter to the kernel.

Instead, let's add a new EFI_DISABLE_RUNTIME boolean Kconfig option, that
would be set to n by default but to y if CONFIG_PREEMPT_RT is enabled.

That way, the current behaviour is preserved but gives users a mechanism
to enable the EFI runtimes services in their kernels if that is required.
For example, if the firmware could guarantee bounded time for EFI calls.

Also, having a separate boolean config could allow users to disable the
EFI runtime services by default even when CONFIG_PREEMPT_RT is not set.
Reported-by: NAlexander Larsson <alexl@redhat.com>
Fixes: d9f283ae ("efi: Disable runtime services on RT")
Signed-off-by: NJavier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20220331151654.184433-1-javierm@redhat.comSigned-off-by: NArd Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/Kconfig

@@ -284,3 +284,18 @@ config EFI_CUSTOM_SSDT_OVERLAYS
 
 	  See Documentation/admin-guide/acpi/ssdt-overlays.rst for more
 	  information.
+
+config EFI_DISABLE_RUNTIME
+	bool "Disable EFI runtime services support by default"
+	default y if PREEMPT_RT
+	help
+	  Allow to disable the EFI runtime services support by default. This can
+	  already be achieved by using the efi=noruntime option, but it could be
+	  useful to have this default without any kernel command line parameter.
+
+	  The EFI runtime services are disabled by default when PREEMPT_RT is
+	  enabled, because measurements have shown that some EFI functions calls
+	  might take too much time to complete, causing large latencies which is
+	  an issue for Real-Time kernels.
+
+	  This default can be overridden by using the efi=runtime option.

drivers/firmware/efi/efi.c

@@ -66,7 +66,7 @@ struct mm_struct efi_mm = {
 
 struct workqueue_struct *efi_rts_wq;
 
-static bool disable_runtime = IS_ENABLED(CONFIG_PREEMPT_RT);
+static bool disable_runtime = IS_ENABLED(CONFIG_EFI_DISABLE_RUNTIME);
 static int __init setup_noefi(char *arg)
 {
 	disable_runtime = true;