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

[Junxiao Chang <junxiao.chang@intel.com>]

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 <junxiao.chang@intel.com>
---
 .../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