On Wed, 04 Aug 2021 09:58:11 +0100,
Oliver Upton <oupton@xxxxxxxxxx> wrote:
>
> Allow userspace to access the guest's virtual counter-timer offset
> through the ONE_REG interface. The value read or written is defined to
> be an offset from the guest's physical counter-timer. Add some
> documentation to clarify how a VMM should use this and the existing
> CNTVCT_EL0.
>
> Signed-off-by: Oliver Upton <oupton@xxxxxxxxxx>
> ---
> Documentation/virt/kvm/api.rst | 10 ++++++++++
> arch/arm64/include/uapi/asm/kvm.h | 1 +
> arch/arm64/kvm/arch_timer.c | 11 +++++++++++
> arch/arm64/kvm/guest.c | 6 +++++-
> include/kvm/arm_arch_timer.h | 1 +
> 5 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 8d4a3471ad9e..28a65dc89985 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -2487,6 +2487,16 @@ arm64 system registers have the following id bit patterns::
> derived from the register encoding for CNTV_CVAL_EL0. As this is
> API, it must remain this way.
>
> +.. warning::
> +
> + The value of KVM_REG_ARM_TIMER_OFFSET is defined as an offset from
> + the guest's view of the physical counter-timer.
> +
> + Userspace should use either KVM_REG_ARM_TIMER_OFFSET or
> + KVM_REG_ARM_TIMER_CVAL to pause and resume a guest's virtual
You probably mean KVM_REG_ARM_TIMER_CNT here, despite the broken
encoding.
> + counter-timer. Mixed use of these registers could result in an
> + unpredictable guest counter value.
> +
> arm64 firmware pseudo-registers have the following bit pattern::
>
> 0x6030 0000 0014 <regno:16>
> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> index b3edde68bc3e..949a31bc10f0 100644
> --- a/arch/arm64/include/uapi/asm/kvm.h
> +++ b/arch/arm64/include/uapi/asm/kvm.h
> @@ -255,6 +255,7 @@ struct kvm_arm_copy_mte_tags {
> #define KVM_REG_ARM_TIMER_CTL ARM64_SYS_REG(3, 3, 14, 3, 1)
> #define KVM_REG_ARM_TIMER_CVAL ARM64_SYS_REG(3, 3, 14, 0, 2)
> #define KVM_REG_ARM_TIMER_CNT ARM64_SYS_REG(3, 3, 14, 3, 2)
> +#define KVM_REG_ARM_TIMER_OFFSET ARM64_SYS_REG(3, 4, 14, 0, 3)
I don't think we can use the encoding for CNTPOFF_EL2 here, as it will
eventually clash with a NV guest using the same feature for its own
purpose. We don't want this offset to overlap with any of the existing
features.
I actually liked your previous proposal of controlling the physical
offset via a device property, as it clearly indicated that you were
dealing with non-architectural state.
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
