Re: [PATCH] KVM: arm/arm64: No need to zero CNTVOFF in kvm_timer_vcpu_put() for VHE

2018-02-22 Thread Marc Zyngier 
On Thu, 22 Feb 2018 08:42:31 +,
Christoffer Dall wrote:
> 
> Hi Shanker,
> 
> On Mon, Feb 19, 2018 at 09:38:07AM -0600, Shanker Donthineni wrote:
> > In AArch64/AArch32, the virtual counter uses a fixed virtual offset
> > of zero in the following situations as per ARMv8 specifications:
> > 
> > 1) HCR_EL2.E2H is 1, and CNTVCT_EL0/CNTVCT are read from EL2.
> > 2) HCR_EL2.{E2H, TGE} is {1, 1}, and either:
> >― CNTVCT_EL0 is read from Non-secure EL0 or EL2.
> >― CNTVCT is read from Non-secure EL0.
> > 
> > So, no need to zero CNTVOFF_EL2/CNTVOFF for VHE case.
> > 
> > Signed-off-by: Shanker Donthineni 
> > ---
> >  virt/kvm/arm/arch_timer.c | 6 --
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> > index 70268c0..86eca324 100644
> > --- a/virt/kvm/arm/arch_timer.c
> > +++ b/virt/kvm/arm/arch_timer.c
> > @@ -541,9 +541,11 @@ void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu)
> >  * The kernel may decide to run userspace after calling vcpu_put, so
> >  * we reset cntvoff to 0 to ensure a consistent read between user
> >  * accesses to the virtual counter and kernel access to the physical
> > -* counter.
> > +* counter of non-VHE case. For VHE, the virtual counter uses a fixed
> > +* virtual offset of zero, so no need to zero CNTVOFF_EL2 register.
> >  */
> > -   set_cntvoff(0);
> > +   if (!has_vhe())
> > +   set_cntvoff(0);
> >  }
> >  
> >  /*
> 
> I'm okay with this change.  I don't think there's a huge gain here
> though.
> 
> Marc, any thoughts or concerns?

None. I don't think this brings any visible performance gain, but hey,
why not.

Acked-by: Marc Zyngier 

M.

-- 
Jazz is not dead, it just smell funny.
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Re: [PATCH] KVM: arm/arm64: No need to zero CNTVOFF in kvm_timer_vcpu_put() for VHE

2018-02-22 Thread Christoffer Dall 
Hi Shanker,

On Mon, Feb 19, 2018 at 09:38:07AM -0600, Shanker Donthineni wrote:
> In AArch64/AArch32, the virtual counter uses a fixed virtual offset
> of zero in the following situations as per ARMv8 specifications:
> 
> 1) HCR_EL2.E2H is 1, and CNTVCT_EL0/CNTVCT are read from EL2.
> 2) HCR_EL2.{E2H, TGE} is {1, 1}, and either:
>— CNTVCT_EL0 is read from Non-secure EL0 or EL2.
>— CNTVCT is read from Non-secure EL0.
> 
> So, no need to zero CNTVOFF_EL2/CNTVOFF for VHE case.
> 
> Signed-off-by: Shanker Donthineni 
> ---
>  virt/kvm/arm/arch_timer.c | 6 --
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index 70268c0..86eca324 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -541,9 +541,11 @@ void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu)
>* The kernel may decide to run userspace after calling vcpu_put, so
>* we reset cntvoff to 0 to ensure a consistent read between user
>* accesses to the virtual counter and kernel access to the physical
> -  * counter.
> +  * counter of non-VHE case. For VHE, the virtual counter uses a fixed
> +  * virtual offset of zero, so no need to zero CNTVOFF_EL2 register.
>*/
> - set_cntvoff(0);
> + if (!has_vhe())
> + set_cntvoff(0);
>  }
>  
>  /*

I'm okay with this change.  I don't think there's a huge gain here
though.

Marc, any thoughts or concerns?

Thanks,
-Christoffer
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[PATCH] KVM: arm/arm64: No need to zero CNTVOFF in kvm_timer_vcpu_put() for VHE

2018-02-19 Thread Shanker Donthineni 
In AArch64/AArch32, the virtual counter uses a fixed virtual offset
of zero in the following situations as per ARMv8 specifications:

1) HCR_EL2.E2H is 1, and CNTVCT_EL0/CNTVCT are read from EL2.
2) HCR_EL2.{E2H, TGE} is {1, 1}, and either:
   — CNTVCT_EL0 is read from Non-secure EL0 or EL2.
   — CNTVCT is read from Non-secure EL0.

So, no need to zero CNTVOFF_EL2/CNTVOFF for VHE case.

Signed-off-by: Shanker Donthineni 
---
 virt/kvm/arm/arch_timer.c | 6 --
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
index 70268c0..86eca324 100644
--- a/virt/kvm/arm/arch_timer.c
+++ b/virt/kvm/arm/arch_timer.c
@@ -541,9 +541,11 @@ void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu)
 * The kernel may decide to run userspace after calling vcpu_put, so
 * we reset cntvoff to 0 to ensure a consistent read between user
 * accesses to the virtual counter and kernel access to the physical
-* counter.
+* counter of non-VHE case. For VHE, the virtual counter uses a fixed
+* virtual offset of zero, so no need to zero CNTVOFF_EL2 register.
 */
-   set_cntvoff(0);
+   if (!has_vhe())
+   set_cntvoff(0);
 }
 
 /*
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Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux 
Foundation Collaborative Project.

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