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

[Paolo Bonzini]

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:
> 
> Â Â Â Â if (vmcs12->cpu_based_vm_exec_control &
> CPU_BASED_USE_TSC_OFFSETING)
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂvmcs_write64(TSC_OFFSET,
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂvcpu->arch.tsc_offset + vmcs12->tsc_offset);
> 
> and here:
> 
> Â Â Â Â vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> 
> and here:
> 
> u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)
> {
> ÂÂÂÂÂÂÂÂreturn 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...

Paolo