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To:     Wei Huang <whuang2@amd.com>, Wei Huang <wei.huang2@amd.com>,
        kvm@vger.kernel.org
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From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by
 VM instructions
Message-ID: <7ea9f2d8-6688-612c-21a8-f3f3517da122@redhat.com>
Date:   Sun, 17 Jan 2021 19:20:29 +0100
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On 15/01/21 08:00, Wei Huang wrote:
> If the whole body inside if-statement is moved out, do you expect the
> interface of x86_emulate_decoded_instruction to be something like:
> 
> int x86_emulate_decoded_instruction(struct kvm_vcpu *vcpu,
>                                      gpa_t cr2_or_gpa,
>                                      int emulation_type, void *insn,
>                                      int insn_len,
>                                      bool write_fault_to_spt)

An idea is to making the body of the new function just

         init_emulate_ctxt(vcpu);

         /*
          * We will reenter on the same instruction since
          * we do not set complete_userspace_io.  This does not
          * handle watchpoints yet, those would be handled in
          * the emulate_ops.
          */
         if (!(emulation_type & EMULTYPE_SKIP) &&
             kvm_vcpu_check_breakpoint(vcpu, &r))
                 return r;

         ctxt->interruptibility = 0;
         ctxt->have_exception = false;
         ctxt->exception.vector = -1;
         ctxt->exception.error_code_valid = false;

         ctxt->perm_ok = false;

         ctxt->ud = emulation_type & EMULTYPE_TRAP_UD;

         r = x86_decode_insn(ctxt, insn, insn_len);

         trace_kvm_emulate_insn_start(vcpu);
         ++vcpu->stat.insn_emulation;
         return r;

because for the new caller, on EMULATION_FAILED you can just re-enter 
the guest.

> And if so, what is the emulation type to use when calling this function
> from svm.c? EMULTYPE_VMWARE_GP?

Just 0 I think.

Paolo