RE: [PATCH -next] x86: Fix unused variable 'msr_val' warning

[Michael Kelley <mikelley@microsoft.com>]

From: Ingo Molnar <mingo.kernel.org@gmail.com> Sent: Monday, March 22, 2021 2:08 PM
> 
> * Xu Yihang <xuyihang@huawei.com> wrote:
> 
> > 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 __maybe_unused should be added.
> >
> > Reference:
> >
> > https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/reference/tlfs
> >
> > Reported-by: Hulk Robot <hulkci@huawei.com>
> > Signed-off-by: Xu Yihang <xuyihang@huawei.com>
> > ---
> >  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