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 18:35 +0200, Paolo Bonzini wrote:
> On 12/04/2018 17:12, KarimAllah Ahmed wrote:
> > 
> > When the TSC MSR is captured while an L2 guest is running then restored,
> > the 'tsc_offset' ends up capturing the L02 TSC_OFFSET instead of the L01
> > TSC_OFFSET. So ensure that this is compensated for when storing the value.
> > 
> > Cc: Jim Mattson <jmattson@google.com>
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Radim Krčmář <rkrcmar@redhat.com>
> > Cc: kvm@vger.kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
> > ---
> >  arch/x86/kvm/vmx.c | 12 +++++++++---
> >  arch/x86/kvm/x86.c |  1 -
> >  2 files changed, 9 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index cff2f50..2f57571 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -2900,6 +2900,8 @@ static u64 guest_read_tsc(struct kvm_vcpu *vcpu)
> >   */
> >  static void vmx_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
> >  {
> > +	u64 l1_tsc_offset = 0;
> > +
> >  	if (is_guest_mode(vcpu)) {
> >  		/*
> >  		 * We're here if L1 chose not to trap WRMSR to TSC. According
> > @@ -2908,16 +2910,20 @@ static void vmx_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
> >  		 * to the newly set TSC to get L2's TSC.
> >  		 */
> >  		struct vmcs12 *vmcs12;
> > +
> >  		/* recalculate vmcs02.TSC_OFFSET: */
> >  		vmcs12 = get_vmcs12(vcpu);
> > -		vmcs_write64(TSC_OFFSET, offset +
> > -			(nested_cpu_has(vmcs12, CPU_BASED_USE_TSC_OFFSETING) ?
> > -			 vmcs12->tsc_offset : 0));
> > +
> > +		l1_tsc_offset = nested_cpu_has(vmcs12, CPU_BASED_USE_TSC_OFFSETING) ?
> > +					vmcs12->tsc_offset : 0;
> > +		vmcs_write64(TSC_OFFSET, offset + l1_tsc_offset);
> >  	} else {
> >  		trace_kvm_write_tsc_offset(vcpu->vcpu_id,
> >  					   vmcs_read64(TSC_OFFSET), offset);
> >  		vmcs_write64(TSC_OFFSET, offset);
> >  	}
> > +
> > +	vcpu->arch.tsc_offset = offset - l1_tsc_offset;
> 
> Using both "offset + l1_tsc_offset" and "offset - l1_tsc_offset" in this 
> function seems wrong to me: if vcpu->arch.tsc_offset must be "offset - 
> l1_tsc_offset", then "offset" must be written to TSC_OFFSET.

Ooops! I forgot to remove the + l1_tsc_offset :D

> 
> I think the bug was introduced by commit 3e3f50262.  Before,
> vmx_read_tsc_offset returned the L02 offset; now it always contains the
> L01 offset.  So the right fix is to adjust vcpu->arch.tsc_offset on
> nested vmentry/vmexit.  If is_guest_mode(vcpu), kvm_read_l1_tsc must use
> a new kvm_x86_ops callback to subtract the L12 offset from the value it
> returns.

ack!

> 
> Thanks,
> 
> Paolo
> 
> > 
> >  }
> >  
> >  /*
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index ac42c85..1a2ed92 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1539,7 +1539,6 @@ EXPORT_SYMBOL_GPL(kvm_read_l1_tsc);
> >  static void kvm_vcpu_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
> >  {
> >  	kvm_x86_ops->write_tsc_offset(vcpu, offset);
> > -	vcpu->arch.tsc_offset = offset;
> >  }
> >  
> >  static inline bool kvm_check_tsc_unstable(void)
> > 
> 
> 
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