Kernel+LibC: Publish a "kernel info page" and use it for gettimeofday()

This patch adds a single "kernel info page" that is mappable read-only
by any process and contains the current time of day.

This is then used to implement a version of gettimeofday() that doesn't
have to make a syscall.

To protect against race condition issues, the info page also has a
serial number which is incremented whenever the kernel updates the
contents of the page. Make sure to verify that the serial number is the
same before and after reading the information you want from the page.

1 files changed, 13 insertions, 3 deletions

diff --git a/Libraries/LibC/time.cpp b/Libraries/LibC/time.cpp
index 4cd2e47fd9..d0d156d67e 100644
--- a/Libraries/LibC/time.cpp
+++ b/Libraries/LibC/time.cpp
@@ -1,10 +1,11 @@
+#include <Kernel/KernelInfoPage.h>
 #include <Kernel/Syscall.h>
 #include <assert.h>
 #include <errno.h>
+#include <string.h>
 #include <sys/time.h>
 #include <sys/times.h>
 #include <time.h>
-#include <string.h>
 
 extern "C" {
 
@@ -21,8 +22,17 @@ time_t time(time_t* tloc)
 
 int gettimeofday(struct timeval* __restrict__ tv, void* __restrict__)
 {
-    int rc = syscall(SC_gettimeofday, tv);
-    __RETURN_WITH_ERRNO(rc, rc, -1);
+    static volatile KernelInfoPage* kernel_info;
+    if (!kernel_info)
+        kernel_info = (volatile KernelInfoPage*)syscall(SC_get_kernel_info_page);
+
+    for (;;) {
+        auto serial = kernel_info->serial;
+        *tv = const_cast<struct timeval&>(kernel_info->now);
+        if (serial == kernel_info->serial)
+            break;
+    }
+    return 0;
 }
 
 char* ctime(const time_t*)