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

[Joerg Roedel <joro@8bytes.org>]

On Sun, Mar 27, 2011 at 12:57:03PM +0200, Avi Kivity wrote:
> 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.

Yes, all machines with tsc-scaling should also have a stable tsc, so
this patch is not strictly required.
I added this patch as a response to Zachary's thoughts about the values
that need to be passed to adjust_tsc_offset. Zachary mentioned that with
this patch-series the adjust_tsc_offset function will either take the
adjustment in guest-tsc or host-tsc units (which is not problematic if
guest and host tsc have the same frequency).
But with tsc-scaling (and Zachary's upcoming patches) this might become
a problem. So I changed the semantic of adjust_tsc_offset to always take
the adjustment in guest-tsc units. This patch-set converts the one and
only call-site of adjust_tsc_offset where the unit is calculated as a
function of the host-tsc.

	Joerg