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

Paolo Bonzini
2021-01-12T14:03:32Z

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