Victor Do Nascimento <victor.donascime...@arm.com> writes:
> At present, Evaluation of both `has_lse2(hwcap)' and
> `has_lse128(hwcap)' may require issuing an `mrs' instruction to query
> a system register. This instruction, when issued from user-space
> results in a trap by the kernel which then returns the value read in
> by the system register. Given the undesirable nature of the
> computational expense associated with the context switch, it is
> important to implement mechanisms to, wherever possible, forgo the
> operation.
>
> In light of this, given how other architectural requirements serving
> as prerequisites have long been assigned HWCAP bits by the kernel, we
> can inexpensively query for their availability before attempting to
> read any system registers. Where one of these early tests fail, we
> can assert that the main feature of interest (be it LSE2 or LSE128)
> cannot be present, allowing us to return from the function early and
> skip the unnecessary expensive kernel-mediated access to system
> registers.
>
> libatomic/ChangeLog:
>
> * config/linux/aarch64/host-config.h (has_lse2): Add test for LSE.
> (has_lse128): Add test for LSE2.
FAOD, the previous OK for this patch still stands.
Thanks,
Richard
> ---
> libatomic/config/linux/aarch64/host-config.h | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/libatomic/config/linux/aarch64/host-config.h
> b/libatomic/config/linux/aarch64/host-config.h
> index 1bc7d839232..4e354124063 100644
> --- a/libatomic/config/linux/aarch64/host-config.h
> +++ b/libatomic/config/linux/aarch64/host-config.h
> @@ -64,8 +64,13 @@ typedef struct __ifunc_arg_t {
> static inline bool
> has_lse2 (unsigned long hwcap, const __ifunc_arg_t *features)
> {
> + /* Check for LSE2. */
> if (hwcap & HWCAP_USCAT)
> return true;
> + /* No point checking further for atomic 128-bit load/store if LSE
> + prerequisite not met. */
> + if (!(hwcap & HWCAP_ATOMICS))
> + return false;
> if (!(hwcap & HWCAP_CPUID))
> return false;
>
> @@ -99,9 +104,11 @@ has_lse128 (unsigned long hwcap, const __ifunc_arg_t
> *features)
> support in older kernels as it is of CPU feature absence. Try fallback
> method to guarantee LSE128 is not implemented.
>
> - In the absence of HWCAP_CPUID, we are unable to check for LSE128. */
> - if (!(hwcap & HWCAP_CPUID))
> - return false;
> + In the absence of HWCAP_CPUID, we are unable to check for LSE128.
> + If feature check available, check LSE2 prerequisite before proceeding.
> */
> + if (!(hwcap & HWCAP_CPUID) || !(hwcap & HWCAP_USCAT))
> + return false;
> +
> unsigned long isar0;
> asm volatile ("mrs %0, ID_AA64ISAR0_EL1" : "=r" (isar0));
> if (AT_FEAT_FIELD (isar0) >= 3)
