[UNIKRAFT PATCH 1/2] arch/x86: Provide ukarch_spinwait pause instruction

[Alexander Jung <a.jung@xxxxxxxxxxx>]

This commit introduces the new global `ukarch_spinwait` for x86
which calls the instruction `pause`.  It's a more intelligent
instruction to call during a spinwait-type operation preventing
over utilization of an x86 CPU and so increases performance.

More details on the instruction can be found at [0].

[0]: https://c9x.me/x86/html/file_module_x86_id_232.html

Signed-off-by: Alexander Jung <alexander.jung@xxxxxxxxx>
---
 arch/x86/x86_64/include/uk/asm/lcpu.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/x86_64/include/uk/asm/lcpu.h 
b/arch/x86/x86_64/include/uk/asm/lcpu.h
index 6a471a1..f1fd463 100644
--- a/arch/x86/x86_64/include/uk/asm/lcpu.h
+++ b/arch/x86/x86_64/include/uk/asm/lcpu.h
@@ -118,4 +118,10 @@ static inline unsigned long ukarch_read_sp(void)
        __asm__ __volatile__("mov %%rsp, %0" : "=r"(sp));
        return sp;
 }
+
+static inline void ukarch_spinwait(void)
+{
+       __asm__ __volatile__("pause" : : : "memory");
+}
+
 #endif /* !__ASSEMBLY__ */
-- 
2.11.0