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

From: Paolo Bonzini <pbonzini@redhat.com>
To: "Raslan, KarimAllah" <karahmed@amazon.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>
Cc: "jmattson@google.com" <jmattson@google.com>,
	"rkrcmar@redhat.com" <rkrcmar@redhat.com>
Subject: Re: [PATCH 1/2] X86/KVM: Properly restore 'tsc_offset' when running an L2 guest
Date: Thu, 12 Apr 2018 22:21:51 +0200	[thread overview]
Message-ID: <ca0e9ac3-1f7b-cefb-a0fe-cc8e6f938c6c@redhat.com> (raw)
In-Reply-To: <1523553662.32594.29.camel@amazon.de>

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...

Paolo