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To:     Wei Huang <wei.huang2@amd.com>, kvm@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, vkuznets@redhat.com,
        seanjc@google.com, joro@8bytes.org, bp@alien8.de,
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        jmattson@google.com, wanpengli@tencent.com, bsd@redhat.com,
        dgilbert@redhat.com, mlevitsk@redhat.com
References: <20210112063703.539893-1-wei.huang2@amd.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by
 VM instructions
Message-ID: <090232a9-7a87-beb9-1402-726bb7cab7e6@redhat.com>
Date:   Tue, 12 Jan 2021 15:01:39 +0100
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On 12/01/21 07:37, Wei Huang wrote:
>   static int gp_interception(struct vcpu_svm *svm)
>   {
>   	struct kvm_vcpu *vcpu = &svm->vcpu;
>   	u32 error_code = svm->vmcb->control.exit_info_1;
> -
> -	WARN_ON_ONCE(!enable_vmware_backdoor);
> +	int rc;
>   
>   	/*
> -	 * VMware backdoor emulation on #GP interception only handles IN{S},
> -	 * OUT{S}, and RDPMC, none of which generate a non-zero error code.
> +	 * Only VMware backdoor and SVM VME errata are handled. Neither of
> +	 * them has non-zero error codes.
>   	 */
>   	if (error_code) {
>   		kvm_queue_exception_e(vcpu, GP_VECTOR, error_code);
>   		return 1;
>   	}
> -	return kvm_emulate_instruction(vcpu, EMULTYPE_VMWARE_GP);
> +
> +	rc = kvm_emulate_instruction(vcpu, EMULTYPE_PARAVIRT_GP);
> +	if (rc > 1)
> +		rc = svm_emulate_vm_instr(vcpu, rc);
> +	return rc;
>   }
>   

Passing back the third byte is quick hacky.  Instead of this change to 
kvm_emulate_instruction, I'd rather check the instruction bytes in 
gp_interception before calling kvm_emulate_instruction.  That would be 
something like:

- move "kvm_clear_exception_queue(vcpu);" inside the "if 
(!(emulation_type & EMULTYPE_NO_DECODE))".  It doesn't apply when you 
are coming back from userspace.

- extract the "if (!(emulation_type & EMULTYPE_NO_DECODE))" body to a 
new function x86_emulate_decoded_instruction.  Call it from 
gp_interception, we know this is not a pagefault and therefore 
vcpu->arch.write_fault_to_shadow_pgtable must be false.

- check ctxt->insn_bytes for an SVM instruction

- if not an SVM instruction, call kvm_emulate_instruction(vcpu, 
EMULTYPE_VMWARE_GP|EMULTYPE_NO_DECODE).

Thanks,

Paolo