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 Krčmář <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.
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.
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)
>
