[PATCH 2/2] x86/irq: Improve local_irq_restore() code generation and performance

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

POPF is a horribly expensive instruction, while STI is an optimised fastpath.

Switching POPF for a conditional branch and STI caused an 8% perf improvement
in various linux measurements.  While I don't expect the change to be that
dramatic in Xen, there will be an improvement.

Furthermore, there is the following code generation improvement:

  add/remove: 0/0 grow/shrink: 3/52 up/down: 52/-966 (-914)

owing to not needing to opencode the restriction to just IF in asm.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
---
 xen/arch/x86/include/asm/system.h | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/include/asm/system.h 
b/xen/arch/x86/include/asm/system.h
index 65e63de69a67..4be235472ecd 100644
--- a/xen/arch/x86/include/asm/system.h
+++ b/xen/arch/x86/include/asm/system.h
@@ -267,13 +267,8 @@ static inline unsigned long 
array_index_mask_nospec(unsigned long index,
 })
 #define local_irq_restore(x)                                     \
 ({                                                               \
-    BUILD_BUG_ON(sizeof(x) != sizeof(long));                     \
-    asm volatile ( "pushfq\n\t"                                  \
-                   "andq %0, (%%rsp)\n\t"                        \
-                   "orq  %1, (%%rsp)\n\t"                        \
-                   "popfq"                                       \
-                   : : "i?r" ( ~X86_EFLAGS_IF ),                 \
-                       "ri" ( (x) & X86_EFLAGS_IF ) );           \
+    if ( (x) & X86_EFLAGS_IF )                                   \
+        local_irq_enable();                                      \
 })
 
 static inline int local_irq_is_enabled(void)
-- 
2.30.2