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Subject: Re: [PATCH v4 1/2] X86/KVM: Properly update 'tsc_offset' to represent
 the running 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: <1523564347-8896-1-git-send-email-karahmed@amazon.de>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Message-ID: <e8f9a996-37dc-f623-2687-92d32175baf0@redhat.com>
Date:   Thu, 12 Apr 2018 22:30:48 +0200
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On 12/04/2018 22:19, KarimAllah Ahmed wrote:
> Update 'tsc_offset' on vmenty/vmexit of L2 guests to ensure that it always
> captures the TSC_OFFSET of the running guest whether it is the L1 or L2
> guest.
> 
> 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
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
> ---
> v1 -> v2:
> 
> - Rewrote the patch to always update tsc_offset to represent the current
>   guest (pbonzini@)

Yes, this is it, thanks.  I'll test it tomorrow and if I have time look
at AMD.

Paolo

> ---
>  arch/x86/include/asm/kvm_host.h |  1 +
>  arch/x86/kvm/vmx.c              | 25 ++++++++++++++++++++-----
>  arch/x86/kvm/x86.c              |  9 ++++++++-
>  3 files changed, 29 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 9fa4f57..3bedfef 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1015,6 +1015,7 @@ struct kvm_x86_ops {
>  
>  	bool (*has_wbinvd_exit)(void);
>  
> +	u64 (*read_l1_tsc_offset)(struct kvm_vcpu *vcpu);
>  	void (*write_tsc_offset)(struct kvm_vcpu *vcpu, u64 offset);
>  
>  	void (*get_exit_info)(struct kvm_vcpu *vcpu, u64 *info1, u64 *info2);
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index cff2f50..9e7dd39 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2895,6 +2895,17 @@ static u64 guest_read_tsc(struct kvm_vcpu *vcpu)
>  	return kvm_scale_tsc(vcpu, host_tsc) + tsc_offset;
>  }
>  
> +static u64 vmx_read_l1_tsc_offset(struct kvm_vcpu *vcpu)
> +{
> +	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> +
> +	if (is_guest_mode(vcpu) &&
> +	    (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETING))
> +		return vcpu->arch.tsc_offset - vmcs12->tsc_offset;
> +
> +	return vcpu->arch.tsc_offset;
> +}
> +
>  /*
>   * writes 'offset' into guest's timestamp counter offset register
>   */
> @@ -11163,11 +11174,8 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>  		vmcs_write64(GUEST_IA32_PAT, vmx->vcpu.arch.pat);
>  	}
>  
> -	if (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETING)
> -		vmcs_write64(TSC_OFFSET,
> -			vcpu->arch.tsc_offset + vmcs12->tsc_offset);
> -	else
> -		vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> +	vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> +
>  	if (kvm_has_tsc_control)
>  		decache_tsc_multiplier(vmx);
>  
> @@ -11469,6 +11477,9 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
>  	if (enable_shadow_vmcs)
>  		copy_shadow_to_vmcs12(vmx);
>  
> +	if (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETING)
> +		vcpu->arch.tsc_offset += vmcs12->tsc_offset;
> +
>  	/*
>  	 * The nested entry process starts with enforcing various prerequisites
>  	 * on vmcs12 as required by the Intel SDM, and act appropriately when
> @@ -12015,6 +12026,9 @@ static void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 exit_reason,
>  
>  	leave_guest_mode(vcpu);
>  
> +	if (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETING)
> +		vcpu->arch.tsc_offset -= vmcs12->tsc_offset;
> +
>  	if (likely(!vmx->fail)) {
>  		if (exit_reason == -1)
>  			sync_vmcs12(vcpu, vmcs12);
> @@ -12688,6 +12702,7 @@ static struct kvm_x86_ops vmx_x86_ops __ro_after_init = {
>  
>  	.has_wbinvd_exit = cpu_has_vmx_wbinvd_exit,
>  
> +	.read_l1_tsc_offset = vmx_read_l1_tsc_offset,
>  	.write_tsc_offset = vmx_write_tsc_offset,
>  
>  	.set_tdp_cr3 = vmx_set_cr3,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ac42c85..3fb1353 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1532,7 +1532,14 @@ static u64 kvm_compute_tsc_offset(struct kvm_vcpu *vcpu, u64 target_tsc)
>  
>  u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)
>  {
> -	return vcpu->arch.tsc_offset + kvm_scale_tsc(vcpu, host_tsc);
> +	u64 tsc_offset;
> +
> +	if (kvm_x86_ops->read_l1_tsc_offset)
> +		tsc_offset = kvm_x86_ops->read_l1_tsc_offset(vcpu);
> +	else
> +		tsc_offset = vcpu->arch.tsc_offset;
> +
> +	return tsc_offset + kvm_scale_tsc(vcpu, host_tsc);
>  }
>  EXPORT_SYMBOL_GPL(kvm_read_l1_tsc);
>  
>