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

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

Hi Junxiao,

On Fri, 31 Jul 2026, at 08:24, Junxiao Chang wrote:
> Add an interface for PREEMPT_RT kernels to dynamically enable or
> disable EFI runtime services.
>
> EFI runtime services are typically disabled on RT systems using
> kernel parameters such as "noefi" or "efi=disable" to avoid
> long latency caused by firmware calls. However, this permanently
> disables EFI runtime services, preventing operations such as UEFI
> firmware updates.
>
> With this change, EFI runtime services can be disabled while
> real-time workloads are running and re-enabled afterwards,
> providing low-latency operation without permanently sacrificing
> firmware functionality.
>
> Signed-off-by: Junxiao Chang <junxiao.chang@intel.com>
> ---
>  drivers/firmware/efi/efi.c              | 31 +++++++++++++++++++++++++
>  drivers/firmware/efi/runtime-wrappers.c | 15 ++++++++++++
>  include/linux/efi.h                     |  1 +
>  3 files changed, 47 insertions(+)
>
> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 0327a39d31fa5..f57784a815c61 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -401,6 +401,32 @@ 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 (efi_runtime_set_enable_flag(enable) != EFI_SUCCESS) {
> +		pr_warn("unable to enable/disable efi runtime service\n");
> +		return -EAGAIN;
> +	}
> +
> +	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 +472,11 @@ static int __init efisubsys_init(void)
>  		goto err_destroy_wq;
>  	}
> 
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && 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();
> diff --git a/drivers/firmware/efi/runtime-wrappers.c 
> b/drivers/firmware/efi/runtime-wrappers.c
> index da8d296216441..8d1554714e3f4 100644
> --- a/drivers/firmware/efi/runtime-wrappers.c
> +++ b/drivers/firmware/efi/runtime-wrappers.c
> @@ -602,3 +602,18 @@ void efi_runtime_assert_lock_held(void)
>  {
>  	WARN_ON(efi_runtime_lock_owner != current);
>  }
> +
> +efi_status_t efi_runtime_set_enable_flag(bool enable)
> +{
> +	if (down_interruptible(&efi_runtime_lock))
> +		return EFI_ABORTED;
> +
> +	if (enable)
> +		set_bit(EFI_RUNTIME_SERVICES, &efi.flags);

This interface allows enabling of EFI_RUNTIME_SERVICES even if it
was disabled for other reasons, e.g., a firmware crash or a 
command line option. IOW, the set_bit() path is only permitted
if the clear_bit() path was taken first.


> +	else
> +		clear_bit(EFI_RUNTIME_SERVICES, &efi.flags);
> +
> +	up(&efi_runtime_lock);
> +
> +	return EFI_SUCCESS;
> +}
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index ccbc35479684a..98b76008fd426 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -1109,6 +1109,7 @@ extern void efi_call_virt_check_flags(unsigned 
> long flags, const void *caller);
>  extern unsigned long efi_call_virt_save_flags(void);
> 
>  void efi_runtime_assert_lock_held(void);
> +efi_status_t efi_runtime_set_enable_flag(bool enable);
> 
>  enum efi_secureboot_mode {
>  	efi_secureboot_mode_unset,
> -- 
> 2.43.0