From: Ingo Molnar <redacted> Sent: Monday, March 22, 2021 2:08 PM

* Xu Yihang [off-list ref] wrote:

quoted

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]

quoted

  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 __maybe_unused should be added.

quoted

Reference:

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

Reported-by: Hulk Robot <redacted>
Signed-off-by: Xu Yihang <redacted>
---
 arch/x86/hyperv/hv_spinlock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/hyperv/hv_spinlock.c b/arch/x86/hyperv/hv_spinlock.c
index f3270c1fc48c..67bc15c7752a 100644
--- a/arch/x86/hyperv/hv_spinlock.c
+++ b/arch/x86/hyperv/hv_spinlock.c

@@ -25,7 +25,7 @@ static void hv_qlock_kick(int cpu)

 static void hv_qlock_wait(u8 *byte, u8 val)
 {
-	unsigned long msr_val;
+	unsigned long msr_val __maybe_unused;
 	unsigned long flags;

Please don't add new __maybe_unused annotations to the x86 tree -
improve the flow instead to help GCC recognize the initialization
sequence better.

Thanks,

	Ingo

Could you elaborate on the thinking here, or point to some written
discussion?   I'm just curious.   In this particular case, it's not a problem
with the flow or gcc detection.  This code really does read an MSR and
ignore that value that is read, so it's not clear how gcc would ever
figure out that's OK.

Michael