Re: [PATCH 1/2] X86/KVM: Properly restore 'tsc_offset' when running an L2 guest

["Raslan, KarimAllah" <karahmed@amazon.de>]

On Thu, 2018-04-12 at 22:21 +0200, Paolo Bonzini wrote:
> On 12/04/2018 19:21, Raslan, KarimAllah wrote:
> > 
> > Now looking further at the code, it seems that everywhere in the code
> > tsc_offset is treated as the L01 TSC_OFFSET.
> > 
> > Like here:
> > 
> >         if (vmcs12->cpu_based_vm_exec_control &
> > CPU_BASED_USE_TSC_OFFSETING)
> >                 vmcs_write64(TSC_OFFSET,
> >                         vcpu->arch.tsc_offset + vmcs12->tsc_offset);
> > 
> > and here:
> > 
> >         vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> > 
> > and here:
> > 
> > u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)
> > {
> >         return vcpu->arch.tsc_offset + kvm_scale_tsc(vcpu, host_tsc);
> > }
> > EXPORT_SYMBOL_GPL(kvm_read_l1_tsc);
> > 
> > ... would not it be simpler and more inline with the current code to
> > just do what I did above + remove the "+ l1_tsc_offset" + probably
> > document tsc_offset ?
> 
> Problem is, I don't think it's correct. :)  A good start would be to try
> disabling MSR_IA32_TSC interception in KVM, prepare a kvm-unit-tests
> test that reads the MSR, and see if you get the host or guest TSC...

I actually just submitted a patch with your original suggestion
(I hope)  because I realized that adjust tsc was still using the wrong
tsc_offset anyway :)

> 
> Paolo
> 
Amazon Development Center Germany GmbH
Berlin - Dresden - Aachen
main office: Krausenstr. 38, 10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B