Subject: Re: [PATCH v7 3/7] KVM: arm64: Allow userspace to configure a vCPU's virtual offset

From: Marc Zyngier <[email-masked]>
Date: Thu, 19 Aug 2021 10:11:09 +0100

On Mon, 16 Aug 2021 01:12:13 +0100,
Oliver Upton <[email-masked]> 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 <[email-masked]>
> Reviewed-by: Andrew Jones <[email-masked]>
> ---
>  Documentation/virt/kvm/api.rst    | 10 ++++++++++
>  arch/arm64/include/uapi/asm/kvm.h |  1 +
>  arch/arm64/kvm/arch_timer.c       | 23 +++++++++++++++++++++++
>  arch/arm64/kvm/guest.c            |  6 +++++-
>  include/kvm/arm_arch_timer.h      |  1 +
>  5 files changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index dae68e68ca23..adb04046a752 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -2463,6 +2463,16 @@ 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_CNT to pause and resume a guest's virtual
> +     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)
>

Andrew, does this warrant an update to the selftest that checks for
sysreg visibility?

I am also wondering how a VMM such as QEMU is going to deal with the
above restriction, given the way it blindly saves/restores all the
registers that KVM exposes, hence hitting that mixed-use that the
documentation warns about...

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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