Re: [RFC: timer passthrough 5/9] KVM: vmx: use tsc_adjust to enable tsc_offset timer passthrough

[Thomas Gleixner]

On Fri, Feb 05 2021 at 13:43, Peter Zijlstra wrote:
> On Fri, Feb 05, 2021 at 06:03:13PM +0800, Zhimin Feng wrote:
>> +static void vmx_adjust_tsc_offset(struct kvm_vcpu *vcpu, bool to_host)
>> +{
>> +	u64 tsc_adjust;
>> +	struct timer_passth_info *local_timer_info;
>> +
>> +	local_timer_info = &per_cpu(passth_info, smp_processor_id());
>> +
>> +	if (to_host) {
>> +		tsc_adjust = local_timer_info->host_tsc_adjust;
>> +		wrmsrl(MSR_IA32_TSC_ADJUST, tsc_adjust);
>> +		vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
>> +	} else {
>> +		rdmsrl(MSR_IA32_TSC_ADJUST, tsc_adjust);
>> +		local_timer_info->host_tsc_adjust = tsc_adjust;
>> +
>> +		wrmsrl(MSR_IA32_TSC_ADJUST, tsc_adjust + vcpu->arch.tsc_offset);
>> +		vmcs_write64(TSC_OFFSET, 0);
>> +	}
>> +}
>
> NAK
>
> This wrecks the host TSC value, any host code between this and actually
> entering that VM will observe batshit time.

VMCS TSC offset is there for a reason...

Thanks,

        tglx