[PATCH] mefi: add dynamic control interface for EFI runtime services

Add an interface to dynamically enable or disable EFI runtime
services at runtime. By default, EFI runtime services remain
enabled, but they can be temporarily disabled to improve real-time
latency.

Currently, EFI runtime services are typically disabled on RT
systems using kernel parameters such as "noefi" or "efi=disable".
However, this permanently disables EFI services, preventing use
cases such as UEFI firmware updates.

With this change, EFI runtime services can be disabled during
execution of real-time workloads and re-enabled afterwards,
allowing better balance between real-time performance and firmware
service availability.

Signed-off-by: Junxiao Chang <[email protected]>
---
 .../admin-guide/kernel-parameters.txt         |  5 ++-
 drivers/firmware/efi/efi.c                    | 39 +++++++++++++++++++
 2 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index b5493a7f8f228..533213101f808 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1595,7 +1595,8 @@ Kernel parameters
 	efi=		[EFI,EARLY]
 			Format: { "debug", "disable_early_pci_dma",
 				  "nochunk", "noruntime", "nosoftreserve",
-				  "novamap", "no_disable_early_pci_dma" }
+				  "novamap", "no_disable_early_pci_dma",
+				  "dynamic" }
 			debug: enable misc debug output.
 			disable_early_pci_dma: disable the busmaster bit on all
 			PCI bridges while in the EFI boot stub.
@@ -1612,6 +1613,8 @@ Kernel parameters
 			novamap: do not call SetVirtualAddressMap().
 			no_disable_early_pci_dma: Leave the busmaster bit set
 			on all PCI bridges while in the EFI boot stub
+			dynamic: enable EFI runtime services, which can be
+			disabled via /sys/firmware/efi/dynamic_enable.
 
 	efi_no_storage_paranoia [EFI,X86,EARLY]
 			Using this parameter you can use more than 50% of
diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 0327a39d31fa5..18d5d99fab821 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -81,6 +81,7 @@ struct mm_struct efi_mm = {
 
 struct workqueue_struct *efi_rts_wq;
 
+static bool efi_in_dynamic __initdata;
 static bool disable_runtime = IS_ENABLED(CONFIG_EFI_DISABLE_RUNTIME);
 static int __init setup_noefi(char *arg)
 {
@@ -115,6 +116,11 @@ static int __init parse_efi_cmdline(char *str)
 	if (parse_option_str(str, "runtime"))
 		disable_runtime = false;
 
+	if (parse_option_str(str, "dynamic")) {
+		disable_runtime = false;
+		efi_in_dynamic = true;
+	}
+
 	if (parse_option_str(str, "nosoftreserve"))
 		set_bit(EFI_MEM_NO_SOFT_RESERVE, &efi.flags);
 
@@ -401,6 +407,34 @@ static void __init efi_debugfs_init(void)
 static inline void efi_debugfs_init(void) {}
 #endif
 
+static ssize_t efi_dynamic_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%d\n", efi_enabled(EFI_RUNTIME_SERVICES));
+}
+
+static ssize_t efi_dynamic_store(struct kobject *kobj, struct kobj_attribute *attr,
+				 const char *buf, size_t count)
+{
+	int ret;
+	bool enable;
+
+	ret = kstrtobool(buf, &enable);
+	if (ret)
+		return ret;
+
+	if (enable)
+		set_bit(EFI_RUNTIME_SERVICES, &efi.flags);
+	else {
+		clear_bit(EFI_RUNTIME_SERVICES, &efi.flags);
+		if (efi_rts_wq)
+			flush_workqueue(efi_rts_wq);
+	}
+
+	return count;
+}
+static struct kobj_attribute efi_dynamic_attr =
+	__ATTR(dynamic_enable, 0644, efi_dynamic_show, efi_dynamic_store);
+
 static int __init efipostcore_init(void)
 {
 	if (!efi_enabled(EFI_RUNTIME_SERVICES))
@@ -446,6 +480,11 @@ static int __init efisubsys_init(void)
 		goto err_destroy_wq;
 	}
 
+	if (efi_in_dynamic && efi.runtime_supported_mask) {
+		if (sysfs_create_file(efi_kobj, &efi_dynamic_attr.attr))
+			pr_warn("unable to register efi dynamic sysfs interface\n");
+	}
+
 	if (efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE |
 				      EFI_RT_SUPPORTED_GET_NEXT_VARIABLE_NAME)) {
 		error = generic_ops_register();
-- 
2.43.0