From 85a8076d08d75cd939136dc01fde4a18aeb78e70 Mon Sep 17 00:00:00 2001
From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Sat, 3 Aug 2019 20:43:16 +0000
Subject: [PATCH] Spin less and more efficiently

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 README.md      | 2 +-
 undocumented.h | 2 ++
 wintun.c       | 9 ++++-----
 3 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/README.md b/README.md
index 186f637..6b2b509 100644
--- a/README.md
+++ b/README.md
@@ -137,7 +137,7 @@ for (;;) {
 }
 ```
 
-It may be desirable to spin for ~50ms before waiting on the `TailMoved` event, in order to reduce latency.
+It may be desirable to spin a short amount of time before waiting on the `TailMoved` event, in order to reduce latency.
 
 When closing the handle, Wintun will set the `Tail` to 0xFFFFFFFF and set the `TailMoved` event to unblock the waiting user process.
 
diff --git a/undocumented.h b/undocumented.h
index c904c7a..cf85823 100644
--- a/undocumented.h
+++ b/undocumented.h
@@ -37,3 +37,5 @@ ZwQuerySystemInformation(
     PVOID SystemInformation,
     ULONG SystemInformationLength,
     ULONG *ReturnLength);
+
+extern NTSTATUS ZwYieldExecution(VOID);
diff --git a/wintun.c b/wintun.c
index 41e1b16..9486457 100644
--- a/wintun.c
+++ b/wintun.c
@@ -402,9 +402,11 @@ _Function_class_(KSTART_ROUTINE)
 static VOID
 TunProcessReceiveData(_Inout_ TUN_CTX *Ctx)
 {
+    KeSetPriorityThread(KeGetCurrentThread(), 1);
+
     TUN_RING *Ring = Ctx->Device.Receive.Ring;
     ULONG RingCapacity = Ctx->Device.Receive.Capacity;
-    const ULONG SpinMax = 10000 * 50 / KeQueryTimeIncrement(); /* 50ms */
+    const ULONG SpinMax = 10000 * 20 / KeQueryTimeIncrement(); /* 20 ms */
     VOID *Events[] = { &Ctx->Device.Disconnected, Ctx->Device.Receive.TailMoved };
     ASSERT(RTL_NUMBER_OF(Events) <= THREAD_WAIT_OBJECTS);
 
@@ -431,10 +433,7 @@ TunProcessReceiveData(_Inout_ TUN_CTX *Ctx)
                 KeQueryTickCount(&SpinNow);
                 if ((ULONG64)SpinNow.QuadPart - (ULONG64)SpinStart.QuadPart >= SpinMax)
                     break;
-
-                /* This should really call KeYieldProcessorEx(&zero), so it does the Hyper-V paravirtualization call,
-                 * but it's not exported. */
-                YieldProcessor();
+                ZwYieldExecution();
             }
             if (RingHead == RingTail)
             {