Re: [PATCH v3 6/8] arm64/efi: Use a mutex to protect the EFI stack and FP/SIMD state

[Will Deacon <will@kernel.org>]

On Thu, Sep 18, 2025 at 12:30:17PM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Replace the spinlock in the arm64 glue code with a mutex, so that
> the CPU can preempted while running the EFI runtime service.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/kernel/efi.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
> index 0d52414415f3..4372fafde8e9 100644
> --- a/arch/arm64/kernel/efi.c
> +++ b/arch/arm64/kernel/efi.c
> @@ -166,15 +166,22 @@ asmlinkage efi_status_t efi_handle_corrupted_x18(efi_status_t s, const char *f)
>  	return s;
>  }
>  
> -static DEFINE_RAW_SPINLOCK(efi_rt_lock);
> +static DEFINE_MUTEX(efi_rt_lock);
>  
>  bool arch_efi_call_virt_setup(void)
>  {
>  	if (!may_use_simd())
>  		return false;
>  
> +	/*
> +	 * This might be called from a non-sleepable context so try to take the
> +	 * lock but don't block on it. This should never fail in practice, as
> +	 * all EFI runtime calls are serialized under the efi_runtime_lock.
> +	 */
> +	if (WARN_ON(!mutex_trylock(&efi_rt_lock)))
> +		return false;

If it will never fail in practice, why do we need the lock at all? Can we
just assert that the efi_runtime_lock is held instead and rely on that?

Will