Re: [PATCH 3/6] KVM: X86: Make tsc_delta calculation a function of guest tsc

[Avi Kivity <avi@redhat.com>]

On 03/25/2011 10:44 AM, Joerg Roedel wrote:
> The calculation of the tsc_delta value to ensure a
> forward-going tsc for the guest is a function of the
> host-tsc. This works as long as the guests tsc_khz is equal
> to the hosts tsc_khz. With tsc-scaling hardware support this
> is not longer true and the tsc_delta needs to be calculated
>
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2126,8 +2126,13 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>   	kvm_x86_ops->vcpu_load(vcpu, cpu);
>   	if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
>   		/* Make sure TSC doesn't go backwards */
> -		s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
> -				native_read_tsc() - vcpu->arch.last_host_tsc;
> +		s64 tsc_delta;
> +		u64 tsc;
> +
> +		kvm_get_msr(vcpu, MSR_IA32_TSC,&tsc);
> +		tsc_delta = !vcpu->arch.last_guest_tsc ? 0 :
> +			     tsc - vcpu->arch.last_guest_tsc;
> +
>   		if (tsc_delta<  0)
>   			mark_tsc_unstable("KVM discovered backwards TSC");
>   		if (check_tsc_unstable()) {


Patch is correct, but can this really happen?  Won't all machines with 
scaled tsc also be stable?

Perhaps not multi-board machines.

-- 
error compiling committee.c: too many arguments to function