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

["Ard Biesheuvel" <ardb@kernel.org>]

(cc Sebastian)

On Sat, 4 Jul 2026, at 02:33, Junxiao Chang wrote:
> 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

I'd prefer not to add a command line option for this. If the sysfs control
is useful, we can enable it unconditionally, or depend on PREEMPT_RT.

> 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);

This is racy, no?


> +		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