Re: [RESEND PATCH 05/13] KVM: x86: Don't attempt VMWare emulation on #GP with non-zero error code

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

> The VMware backdoor hooks #GP faults on IN{S}, OUT{S}, and RDPMC, none
> of which generate a non-zero error code for their #GP.  Re-injecting #GP
> instead of attempting emulation on a non-zero error code will allow a
> future patch to move #GP injection (for emulation failure) into
> kvm_emulate_instruction() without having to plumb in the error code.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

(I just need to get this off my chest)

There was a long-standing issue with #GP interception: in case the
exception has nothing to do with VMware we were getting into infinite
loop of #GPs (and not #GP -> #DF -> #TF), e.g. here is a trace of 
platform_info selftest:

           <...>-43752 [001]  3615.602298: kvm_exit:             reason EXIT_MSR rip 0x4015c2 info 0 0
           <...>-43752 [001]  3615.602299: kvm_msr:              msr_read ce = 0x0 (#GP)
           <...>-43752 [001]  3615.602300: kvm_inj_exception:    #GP (0x0)
           <...>-43752 [001]  3615.602301: kvm_entry:            vcpu 0
           <...>-43752 [001]  3615.602302: kvm_exit:             reason EXIT_EXCP_GP rip 0x4015c2 info 6a 0
           <...>-43752 [001]  3615.602308: kvm_emulate_insn:     0:4015c2: 0f 32
           <...>-43752 [001]  3615.602308: kvm_inj_exception:    #GP (0x6a)
           <...>-43752 [001]  3615.602309: kvm_entry:            vcpu 0
           <...>-43752 [001]  3615.602310: kvm_exit:             reason EXIT_EXCP_GP rip 0x4015c2 info 6a 0
           <...>-43752 [001]  3615.602312: kvm_emulate_insn:     0:4015c2: 0f 32
           <...>-43752 [001]  3615.602312: kvm_inj_exception:    #GP (0x6a)
           <...>-43752 [001]  3615.602313: kvm_entry:            vcpu 0
  and so on.

This commit fixes the issue as the second #GP has error code:

           <...>-52213 [006]  3740.739495: kvm_entry:            vcpu 0
           <...>-52213 [006]  3740.739496: kvm_exit:             reason EXIT_MSR rip 0x4015c2 info 0 0
           <...>-52213 [006]  3740.739497: kvm_msr:              msr_read ce = 0x0 (#GP)
           <...>-52213 [006]  3740.739502: kvm_inj_exception:    #GP (0x0)
           <...>-52213 [006]  3740.739503: kvm_entry:            vcpu 0
           <...>-52213 [006]  3740.739504: kvm_exit:             reason EXIT_EXCP_GP rip 0x4015c2 info 6a 0
           <...>-52213 [006]  3740.739505: kvm_inj_exception:    #DF (0x0)
           <...>-52213 [006]  3740.739506: kvm_entry:            vcpu 0
           <...>-52213 [006]  3740.739507: kvm_exit:             reason EXIT_EXCP_GP rip 0x4015c2 info 42 0
           <...>-52213 [006]  3740.739508: kvm_fpu:              unload
           <...>-52213 [006]  3740.739510: kvm_userspace_exit:   reason KVM_EXIT_SHUTDOWN (8)

I'm not exactly sure this covers all possible cases as there might be
other cases when error code is not set but this is definitely an
improvement.

Reviewed-and-tested-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

> ---
>  arch/x86/kvm/svm.c     | 6 +++++-
>  arch/x86/kvm/vmx/vmx.c | 7 ++++++-
>  2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 5a42f9c70014..b96a119690f4 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -2772,11 +2772,15 @@ static int gp_interception(struct vcpu_svm *svm)
>  
>  	WARN_ON_ONCE(!enable_vmware_backdoor);
>  

In case you'll be respinning for whatever reason, could you please add a
short comment here (and vmx) saying something like "#GP interception for
VMware backdoor emulation only handles IN{S}, OUT{S}, and RDPMC and none
of these have a non-zero error code set" (I don't like the fact that
we'll need to have two copies of it but I can't think of a better place
for it).

> +	if (error_code) {
> +		kvm_queue_exception_e(vcpu, GP_VECTOR, error_code);
> +		return 1;
> +	}
>  	er = kvm_emulate_instruction(vcpu, EMULTYPE_VMWARE);
>  	if (er == EMULATE_USER_EXIT)
>  		return 0;
>  	else if (er != EMULATE_DONE)
> -		kvm_queue_exception_e(vcpu, GP_VECTOR, error_code);
> +		kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
>  	return 1;
>  }
>  
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 6ecf773825e2..3ee0dd304bc7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4509,11 +4509,16 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>  
>  	if (!vmx->rmode.vm86_active && is_gp_fault(intr_info)) {
>  		WARN_ON_ONCE(!enable_vmware_backdoor);
> +
> +		if (error_code) {
> +			kvm_queue_exception_e(vcpu, GP_VECTOR, error_code);
> +			return 1;
> +		}
>  		er = kvm_emulate_instruction(vcpu, EMULTYPE_VMWARE);
>  		if (er == EMULATE_USER_EXIT)
>  			return 0;
>  		else if (er != EMULATE_DONE)
> -			kvm_queue_exception_e(vcpu, GP_VECTOR, error_code);
> +			kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
>  		return 1;
>  	}

-- 
Vitaly