Re: [PATCH 3/4] [PATCH] kvmclock: allow it to be turned off

[Avi Kivity <avi@qumranet.com>]

Glauber Costa wrote:
> Use the lower 3 lower bits of the system time msr to turn off the clock.
> This means that all clock registration has to be aligned in a 4-byte boundary
>
>   

3 bits -> 8 bytes.

How about just using just bit 0 as an enable bit (not a disable bit).  
That means the default value of zero means the clock is disabled, and 
that we have a couple of more bits to enable future features.

> Signed-off-by: Glauber Costa <gcosta@redhat.com>
> ---
>  arch/x86/kvm/x86.c |    5 +++++
>  1 files changed, 5 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6abd784..7ce14ce 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -591,6 +591,11 @@ int kvm_set_msr_common(struct kvm_vcpu *
>  		if (vcpu->arch.time_page)
>  			kvm_release_page_dirty(vcpu->arch.time_page);
>  
> +		/* 4-byte unaligned accesses are invalid */
> +		if (data & 0x7) {
> +			vcpu->arch.time_page = NULL;
> +			break;
> +		}
>  		vcpu->arch.time = data & PAGE_MASK;
>  		vcpu->arch.time_offset = data & ~PAGE_MASK;
>  
>   


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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