From 3d65188e545bbd9c6907212161b35158fb12e5e9 Mon Sep 17 00:00:00 2001
From: Zhao Yakui <yakui.zhao@intel.com>
Date: Thu, 18 Jul 2019 16:16:08 +0800
Subject: [PATCH] HV: Refine the usage of monitor/mwait to avoid the possible
 lockup

Based on SDM Vol2 the monitor uses the RAX register to setup the address
monitored by HW. The mwait uses the rax/rcx as the hints that the process
will enter. It is incorrect that the same value is used for monitor/mwait.
The ecx in mwait specifies the optional externsions.

At the same time it needs to check whether the the value of monitored addr
is already expected before entering mwait. Otherwise it will have possible
lockup.

V1->V2: Add the asm wrappper of monitor/mwait to avoid the mixed usage of
inline assembly in wait_sync_change

v2-v3: Remove the unnecessary line break in asm_monitor/asm_mwait.
       Follow Fei's comment to remove the mwait ecx hint setting that
treats the interrupt as break event. It only needs to check whether the
value of psync_change is already expected.

Tracked-On: #3442
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>
---
 hypervisor/arch/x86/cpu.c | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/hypervisor/arch/x86/cpu.c b/hypervisor/arch/x86/cpu.c
index ee9f70e53..9d5f8082f 100644
--- a/hypervisor/arch/x86/cpu.c
+++ b/hypervisor/arch/x86/cpu.c
@@ -415,21 +415,29 @@ static void print_hv_banner(void)
 	printf(boot_msg);
 }
 
+static
+inline void asm_monitor(const uint64_t *addr, uint64_t ecx, uint64_t edx)
+{
+	asm volatile("monitor\n" : : "a" (addr), "c" (ecx), "d" (edx));
+}
+
+static
+inline void asm_mwait(uint64_t eax, uint64_t ecx)
+{
+	asm volatile("mwait\n" : : "a" (eax), "c" (ecx));
+}
+
 /* wait until *sync == wake_sync */
 void wait_sync_change(uint64_t *sync, uint64_t wake_sync)
 {
 	if (has_monitor_cap()) {
 		/* Wait for the event to be set using monitor/mwait */
-		asm volatile ("1: cmpq      %%rbx,(%%rax)\n"
-			      "   je        2f\n"
-			      "   monitor\n"
-			      "   mwait\n"
-			      "   jmp       1b\n"
-			      "2:\n"
-			      :
-			      : "a" (sync), "d"(0), "c"(0),
-			      "b"(wake_sync)
-			      : "cc");
+		while (*((volatile uint64_t *)sync) != wake_sync) {
+			asm_monitor(sync, 0UL, 0UL);
+			if (*((volatile uint64_t *)sync) != wake_sync) {
+				asm_mwait(0UL, 0UL);
+			}
+		}
 	} else {
 		/* Wait for the event to be set using pause */
 		asm volatile ("1: cmpq      %%rbx,(%%rax)\n"