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Subject: Re: [PATCH 1/2] X86/KVM: Properly restore 'tsc_offset' when running
 an L2 guest
To:     KarimAllah Ahmed <karahmed@amazon.de>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc:     Jim Mattson <jmattson@google.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
References: <1523545958-28059-1-git-send-email-karahmed@amazon.de>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Message-ID: <d03d253a-d184-4c95-91b2-45e94659b9bd@redhat.com>
Date:   Thu, 12 Apr 2018 18:35:25 +0200
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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.

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)
>