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 <jmatt...@google.com>
> > Cc: Paolo Bonzini <pbonz...@redhat.com>
> > Cc: Radim Krčmář <rkrc...@redhat.com>
> > Cc: k...@vger.kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Signed-off-by: KarimAllah Ahmed <karah...@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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