2018-02-19 15:39:51

by Shanker Donthineni

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

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 <[email protected]>
---
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);
}

/*
--
Qualcomm Datacenter Technologies, Inc. on behalf of the Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.

2018-02-22 08:43:28

by Christoffer Dall

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

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 <[email protected]>
> ---
> 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

2018-02-22 08:59:15

by Marc Zyngier

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

On Thu, 22 Feb 2018 08:42:31 +0000,
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:
> > $B!=(B CNTVCT_EL0 is read from Non-secure EL0 or EL2.
> > $B!=(B CNTVCT is read from Non-secure EL0.
> >
> > So, no need to zero CNTVOFF_EL2/CNTVOFF for VHE case.
> >
> > Signed-off-by: Shanker Donthineni <[email protected]>
> > ---
> > 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 <[email protected]>

M.

--
Jazz is not dead, it just smell funny.