[PATCH] arm64/fpsimd: Avoid unnecessary per-CPU buffers for EFI runtime calls

[Mark Brown]

On Tue, Mar 18, 2025 at 02:24:22PM +0100, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb at kernel.org>
> 
> The EFI specification has some elaborate rules about which runtime
> services may be called while another runtime service call is already in
> progress. In Linux, however, for simplicity, all EFI runtime service
> invocations are serialized via the efi_runtime_lock semaphore.

> This implies that calls to the helper pair arch_efi_call_virt_setup()
> and arch_efi_call_virt_teardown() are serialized too, and are guaranteed
> not to nest.  Furthermore, the arm64 arch code has its own spinlock to
> serialize use of the EFI runtime stack, of which only a single instance
> exists.

> This all means that the FP/SIMD and SVE state preserve/restore logic in
> __efi_fpsimd_begin() and __efi_fpsimd_end() are also serialized, and
> only a single instance of the associated per-CPU variables can ever be
> in use at the same time. There is therefore no need at all for per-CPU
> variables here, and they can all be replaced with singleton instances.
> This saves a non-trivial amount of memory on systems with many CPUs.

That makes life a lot easier all round, and like you say the memory
savings are nice.

Reviewed-by: Mark Brown <broonie at kernel.org>

> -		if (system_supports_sve() && likely(efi_sve_state)) {
> -			char *sve_state = this_cpu_ptr(efi_sve_state);
> +		if (system_supports_sve() && efi_sve_state != NULL) {

The change does remove a bunch of these likely() annotations, though the
chances that they were ever having an impact seem minimal.
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