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

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

[Xu Yihang]

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;
 
 	if (in_nmi())
-- 
2.17.1

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

[Wei Liu]

On Mon, Mar 22, 2021 at 11:17:13AM +0800, Xu Yihang 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>

I modified the commit message a bit and queued this up for hyperv-next.
Thanks.

Wei.

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

[Ingo Molnar]

* 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

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

[Michael Kelley]

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

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

[Ingo Molnar]

* Michael Kelley <mikelley@microsoft.com> wrote:

> 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.

Yeah, so the canonical way to signal that the msr_val isn't used would 
be to rewrite this as:


	if (READ_ONCE(*byte) == val) {
		unsigned long msr_val;

		rdmsrl(HV_X64_MSR_GUEST_IDLE, msr_val);

		(void)msr_val;
	}

(Also see the patch below - untested.)

This makes it abundantly clear that the rdmsr() msr_val return value 
is not 'maybe' unused, but totally intentionally skipped.

Thanks,

	Ingo

 arch/x86/hyperv/hv_spinlock.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/hyperv/hv_spinlock.c b/arch/x86/hyperv/hv_spinlock.c
index f3270c1fc48c..7d948513ed42 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 +47,14 @@ 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);
 }
 

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

[Xu Yihang]

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@huawei.com>
Signed-off-by: Xu Yihang <xuyihang@huawei.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

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

[Wei Liu]

On Tue, Mar 23, 2021 at 01:13:03AM +0100, Ingo Molnar wrote:
> 
> * Michael Kelley <mikelley@microsoft.com> wrote:
> 
> > 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.
> 
> Yeah, so the canonical way to signal that the msr_val isn't used would 
> be to rewrite this as:
> 
> 
> 	if (READ_ONCE(*byte) == val) {
> 		unsigned long msr_val;
> 
> 		rdmsrl(HV_X64_MSR_GUEST_IDLE, msr_val);
> 
> 		(void)msr_val;
> 	}
> 
> (Also see the patch below - untested.)
> 
> This makes it abundantly clear that the rdmsr() msr_val return value 
> is not 'maybe' unused, but totally intentionally skipped.
> 
> Thanks,
> 
> 	Ingo
> 

Thank you for the advice, Ingo.

Wei.

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

[Wei Liu]

On Tue, Mar 23, 2021 at 10:43:02AM +0800, Xu Yihang 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 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@huawei.com>
> Signed-off-by: Xu Yihang <xuyihang@huawei.com>

Applied to hyperv-next. Thanks.

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

[Michael Kelley]

From: Xu Yihang <xuyihang@huawei.com> Sent: Thursday, March 18, 2021 1:04 AM
> 
> 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;
> 
> 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;
> 
>  	if (in_nmi())
> --
> 2.17.1

This is one of those slightly weird cases where the side 
effect of reading the synthetic MSR provided by Hyper-V
is what we want.  The returned value is irrelevant and
never used, so the __maybe_unused annotation is correct.

Let me suggest updating the commit message, and adding
a brief comment to the code to explain this.  The side
effect behavior is described in the Hyper-V TLFS, though
perhaps not as clearly as it should be.

Michael

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

[Xu Yihang]

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;

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;
 
 	if (in_nmi())
-- 
2.17.1