[U-Boot] [PATCH 1/2] x86: Add a 'pause' instruction in __udelay() for QEMU target

[Bin Meng]

From: Miao Yan <yanmiaobest at gmail.com>

When running SMP configuration on QEMU (tcg mode, no kvm), there is
a busy loop in start_aps(), calling udelay(), that waits for APs to
show up online. However, there is a chance that VCPU1 will be timeout
waiting, IOW the secondary VCPUs haven't started their execution yet.

This patch adds a 'pause' instruction in __udelay() only for QEMU
target, to give other VCPUs a chance to run. When QEMU sees the
'pause' instruction, it will yeild the execution to other CPUs.

Signed-off-by: Miao Yan <yanmiaobest at gmail.com>
Signed-off-by: Bin Meng <bmeng.cn at gmail.com>
---

 arch/x86/lib/tsc_timer.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/lib/tsc_timer.c b/arch/x86/lib/tsc_timer.c
index 7f5ba2c..0df1af2 100644
--- a/arch/x86/lib/tsc_timer.c
+++ b/arch/x86/lib/tsc_timer.c
@@ -355,7 +355,15 @@ void __udelay(unsigned long usec)
 	stop = now + usec * get_tbclk_mhz();
 
 	while ((int64_t)(stop - get_ticks()) > 0)
+#if defined(CONFIG_QEMU) && defined(CONFIG_SMP)
+		/*
+		 * Add a 'pause' instruction on qemu target,
+		 * to give other VCPUs a chance to run.
+		 */
+		asm volatile("pause");
+#else
 		;
+#endif
 }
 
 int timer_init(void)
-- 
1.8.2.1