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

From: Raslan, KarimAllah
Date: Thu Apr 12 2018 - 13:21:14 EST


On Thu, 2018-04-12 at 17:04 +0000, Raslan, KarimAllah wrote:
> 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@xxxxxxxxxx>
> > > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > > Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> > > Cc: kvm@xxxxxxxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
> > > ---
> > > 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!

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)
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂvmcs_write64(TSC_OFFSET,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂvcpu->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)
{
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 ?

>
> >
> >
> > 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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