linux-kernel - Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by VM instructions

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Date:   Fri, 15 Jan 2021 01:00:22 -0600
From:   Wei Huang <whuang2@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Wei Huang <wei.huang2@....com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, vkuznets@...hat.com,
        seanjc@...gle.com, joro@...tes.org, bp@...en8.de,
        tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
        jmattson@...gle.com, wanpengli@...cent.com, bsd@...hat.com,
        dgilbert@...hat.com, mlevitsk@...hat.com
Subject: Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by
 VM instructions



On 1/12/21 8:01 AM, Paolo Bonzini wrote:
> 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.

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)

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

> 
> - 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
>