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 <[email protected]>
Signed-off-by: Xu Yihang <[email protected]>
---
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
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 <[email protected]>
> Signed-off-by: Xu Yihang <[email protected]>
I modified the commit message a bit and queued this up for hyperv-next.
Thanks.
Wei.
* Xu Yihang <[email protected]> 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 <[email protected]>
> Signed-off-by: Xu Yihang <[email protected]>
> ---
> 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
From: Ingo Molnar <[email protected]> Sent: Monday, March 22, 2021 2:08 PM
>
> * Xu Yihang <[email protected]> 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 <[email protected]>
> > Signed-off-by: Xu Yihang <[email protected]>
> > ---
> > 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
* Michael Kelley <[email protected]> wrote:
> From: Ingo Molnar <[email protected]> Sent: Monday, March 22, 2021 2:08 PM
> >
> > * Xu Yihang <[email protected]> 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 <[email protected]>
> > > Signed-off-by: Xu Yihang <[email protected]>
> > > ---
> > > 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);
}
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 <[email protected]>
Signed-off-by: Xu Yihang <[email protected]>
---
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
On Tue, Mar 23, 2021 at 01:13:03AM +0100, Ingo Molnar wrote:
>
> * Michael Kelley <[email protected]> wrote:
>
> > From: Ingo Molnar <[email protected]> Sent: Monday, March 22, 2021 2:08 PM
> > >
> > > * Xu Yihang <[email protected]> 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 <[email protected]>
> > > > Signed-off-by: Xu Yihang <[email protected]>
> > > > ---
> > > > 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.
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 <[email protected]>
> Signed-off-by: Xu Yihang <[email protected]>
Applied to hyperv-next. Thanks.