linux-kernel - [PATCH -next] x86: Fix unused variable 'msr

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Date:   Tue, 23 Mar 2021 10:43:02 +0800
From:   Xu Yihang <xuyihang@...wei.com>
To:     <kys@...rosoft.com>, <haiyangz@...rosoft.com>,
        <sthemmin@...rosoft.com>, <tglx@...utronix.de>, <mingo@...hat.com>,
        <bp@...en8.de>, <x86@...nel.org>
CC:     <linux-hyperv@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <xuyihang@...wei.com>, <johnny.chenyi@...wei.com>,
        <heying24@...wei.com>
Subject: [PATCH -next] x86: Fix unused variable 'msr_val' warning

Fixes the following W=1 kernel build warning(s):
arch/x86/hyperv/hv_spinlock.c:28:16: warning: variable ‘msr_val’ set but not used [-Wunused-but-set-variable]
  unsigned long msr_val;

As Hypervisor Top-Level Functional Specification states in chapter 7.5 Virtual Processor Idle Sleep State, "A partition which possesses the AccessGuestIdleMsr privilege (refer to section 4.2.2) may trigger entry into the virtual processor idle sleep state through a read to the hypervisor-defined MSR HV_X64_MSR_GUEST_IDLE". That means only a read is necessary, msr_val is not uesed, so potentially cast to void in order to silent this warning.

Reference:
https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/reference/tlfs

Reported-by: Hulk Robot <hulkci@...wei.com>
Signed-off-by: Xu Yihang <xuyihang@...wei.com>
---
 arch/x86/hyperv/hv_spinlock.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/hyperv/hv_spinlock.c b/arch/x86/hyperv/hv_spinlock.c
index f3270c1fc48c..91cfe698bde0 100644
--- a/arch/x86/hyperv/hv_spinlock.c
+++ b/arch/x86/hyperv/hv_spinlock.c
@@ -25,7 +25,6 @@ static void hv_qlock_kick(int cpu)
 
 static void hv_qlock_wait(u8 *byte, u8 val)
 {
-	unsigned long msr_val;
 	unsigned long flags;
 
 	if (in_nmi())
@@ -48,8 +47,13 @@ static void hv_qlock_wait(u8 *byte, u8 val)
 	/*
 	 * Only issue the rdmsrl() when the lock state has not changed.
 	 */
-	if (READ_ONCE(*byte) == val)
+	if (READ_ONCE(*byte) == val) {
+		unsigned long msr_val;
+
 		rdmsrl(HV_X64_MSR_GUEST_IDLE, msr_val);
+
+		(void)msr_val;
+	}
 	local_irq_restore(flags);
 }
 
-- 
2.17.1