[PATCH v4 4/4] aarch64: Add explicit checks for implicit LSE/LSE2 requirements.

[Victor Do Nascimento]

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.
---
 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)
-- 
2.42.0