linux-kernel - Re: [PATCH] KVM: nVMX: remove side effects from nested_vmx_exit

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Date:   Wed, 18 Mar 2020 11:52:47 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     sean.j.christopherson@...el.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: remove side effects from nested_vmx_exit_reflected

Paolo Bonzini <pbonzini@...hat.com> writes:

> The name of nested_vmx_exit_reflected suggests that it's purely
> a test, but it actually marks VMCS12 pages as dirty.  Move this to
> vmx_handle_exit, observing that the initial nested_run_pending check in
> nested_vmx_exit_reflected is pointless---nested_run_pending has just
> been cleared in vmx_vcpu_run and won't be set until handle_vmlaunch
> or handle_vmresume.
>
> Suggested-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 18 ++----------------
>  arch/x86/kvm/vmx/nested.h |  1 +
>  arch/x86/kvm/vmx/vmx.c    | 19 +++++++++++++++++--
>  3 files changed, 20 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 8578513907d7..4ff859c99946 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3527,7 +3527,7 @@ static void vmcs12_save_pending_event(struct kvm_vcpu *vcpu,
>  }
>  
>  
> -static void nested_mark_vmcs12_pages_dirty(struct kvm_vcpu *vcpu)
> +void nested_mark_vmcs12_pages_dirty(struct kvm_vcpu *vcpu)
>  {
>  	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>  	gfn_t gfn;
> @@ -5543,8 +5543,7 @@ bool nested_vmx_exit_reflected(struct kvm_vcpu *vcpu, u32 exit_reason)
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>  	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>  
> -	if (vmx->nested.nested_run_pending)
> -		return false;
> +	WARN_ON_ONCE(vmx->nested.nested_run_pending);
>  
>  	if (unlikely(vmx->fail)) {
>  		trace_kvm_nested_vmenter_failed(
> @@ -5553,19 +5552,6 @@ bool nested_vmx_exit_reflected(struct kvm_vcpu *vcpu, u32 exit_reason)
>  		return true;
>  	}
>  
> -	/*
> -	 * The host physical addresses of some pages of guest memory
> -	 * are loaded into the vmcs02 (e.g. vmcs12's Virtual APIC
> -	 * Page). The CPU may write to these pages via their host
> -	 * physical address while L2 is running, bypassing any
> -	 * address-translation-based dirty tracking (e.g. EPT write
> -	 * protection).
> -	 *
> -	 * Mark them dirty on every exit from L2 to prevent them from
> -	 * getting out of sync with dirty tracking.
> -	 */
> -	nested_mark_vmcs12_pages_dirty(vcpu);
> -
>  	trace_kvm_nested_vmexit(kvm_rip_read(vcpu), exit_reason,
>  				vmcs_readl(EXIT_QUALIFICATION),
>  				vmx->idt_vectoring_info,
> diff --git a/arch/x86/kvm/vmx/nested.h b/arch/x86/kvm/vmx/nested.h
> index 21d36652f213..f70968b76d33 100644
> --- a/arch/x86/kvm/vmx/nested.h
> +++ b/arch/x86/kvm/vmx/nested.h
> @@ -33,6 +33,7 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 exit_reason,
>  int get_vmx_mem_address(struct kvm_vcpu *vcpu, unsigned long exit_qualification,
>  			u32 vmx_instruction_info, bool wr, int len, gva_t *ret);
>  void nested_vmx_pmu_entry_exit_ctls_update(struct kvm_vcpu *vcpu);
> +void nested_mark_vmcs12_pages_dirty(struct kvm_vcpu *vcpu);
>  bool nested_vmx_check_io_bitmaps(struct kvm_vcpu *vcpu, unsigned int port,
>  				 int size);
>  
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index b447d66f44e6..07299a957d4a 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5851,8 +5851,23 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
>  	if (vmx->emulation_required)
>  		return handle_invalid_guest_state(vcpu);
>  
> -	if (is_guest_mode(vcpu) && nested_vmx_exit_reflected(vcpu, exit_reason))
> -		return nested_vmx_reflect_vmexit(vcpu, exit_reason);
> +	if (is_guest_mode(vcpu)) {
> +		/*
> +		 * The host physical addresses of some pages of guest memory
> +		 * are loaded into the vmcs02 (e.g. vmcs12's Virtual APIC
> +		 * Page). The CPU may write to these pages via their host
> +		 * physical address while L2 is running, bypassing any
> +		 * address-translation-based dirty tracking (e.g. EPT write
> +		 * protection).
> +		 *
> +		 * Mark them dirty on every exit from L2 to prevent them from
> +		 * getting out of sync with dirty tracking.
> +		 */
> +		nested_mark_vmcs12_pages_dirty(vcpu);
> +
> +		if (nested_vmx_exit_reflected(vcpu, exit_reason))
> +			return nested_vmx_reflect_vmexit(vcpu, exit_reason);
> +	}
>  
>  	if (exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY) {
>  		dump_vmcs();

The only functional difference seems to be that we're now doing
nested_mark_vmcs12_pages_dirty() in vmx->fail case too and this seems
superfluous: we failed to enter L2 so 'special' pages should remain
intact (right?) but this should be an uncommon case.

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly