Re: [kvm-devel] [RFC][PATCH] svm.c: tsc unsigned delta calculation

[Joerg R. <joe...@am...>]

On Wed, May 07, 2008 at 05:01:02PM -0500, Ryan Harper wrote:
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 5528121..c919ddd 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -685,8 +685,14 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  		 * increasing TSC.
>  		 */
>  		rdtscll(tsc_this);
> -		delta = vcpu->arch.host_tsc - tsc_this;
> -		svm->vmcb->control.tsc_offset += delta;
> +		/* we only need to adjust this if the old tsc was ahead
> +		* also, we'll generate a massively large u64 value if
> +		* tsc_this is less than host_tsc because of unsigned math
> +		*/
> +		if (tsc_this < vcpu->arch.host_tsc) {
> +			delta = vcpu->arch.host_tsc - tsc_this;
> +			svm->vmcb->control.tsc_offset += delta;
> +		}
>  		vcpu->cpu = cpu;
>  		kvm_migrate_apic_timer(vcpu);
>  	}

Hmm, I think this can result in inaccurate guest time because it makes
the tsc hopping. Does it fix the problem when you make delta an s64?

Joerg

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