Re: [PATCH 1/7] KVM: TDX: Add a place holder to handle TDX VM exit

[Xiaoyao Li]

On 12/16/2024 8:54 AM, Binbin Wu wrote:
> On 12/13/2024 4:57 PM, Xiaoyao Li wrote:
> > On 12/1/2024 11:53 AM, Binbin Wu wrote:
...
> > >   }
> > >     void tdx_load_mmu_pgd(struct kvm_vcpu *vcpu, hpa_t root_hpa, int 
> > > pgd_level)
> > > @@ -1135,6 +1215,88 @@ int tdx_sept_remove_private_spte(struct kvm 
> > > *kvm, gfn_t gfn,
> > >       return tdx_sept_drop_private_spte(kvm, gfn, level, pfn);
> > >   }
> > >   +int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
> > > +{
> > > +    struct vcpu_tdx *tdx = to_tdx(vcpu);
> > > +    u64 vp_enter_ret = tdx->vp_enter_ret;
> > > +    union vmx_exit_reason exit_reason;
> > > +
> > > +    if (fastpath != EXIT_FASTPATH_NONE)
> > > +        return 1;
> > > +
> > > +    /*
> > > +     * Handle TDX SW errors, including TDX_SEAMCALL_UD, 
> > > TDX_SEAMCALL_GP and
> > > +     * TDX_SEAMCALL_VMFAILINVALID.
> > > +     */
> > > +    if (unlikely((vp_enter_ret & TDX_SW_ERROR) == TDX_SW_ERROR)) {
> > > +        KVM_BUG_ON(!kvm_rebooting, vcpu->kvm);
> > > +        goto unhandled_exit;
> > > +    }
> > > +
> > > +    /*
> > > +     * Without off-TD debug enabled, failed_vmentry case must have
> > > +     * TDX_NON_RECOVERABLE set.
> > > +     */
> >
> > This comment is confusing. I'm not sure why it is put here. Below code 
> > does nothing with exit_reason.failed_vmentry.
>
> Because when failed_vmentry occurs, vp_enter_ret will have
> TDX_NON_RECOVERABLE set, so it will be handled below.

The words somehow is confusing, which to me is implying something like:

	WARN_ON(!debug_td() && exit_reason.failed_vmentry &&
                !(vp_enter_ret & TDX_NON_RECOVERABLE))

Besides, VMX returns KVM_EXIT_FAIL_ENTRY for vm-entry failure. So the 
question is why TDX cannot do it same way?

> > > +    if (unlikely(vp_enter_ret & (TDX_ERROR | TDX_NON_RECOVERABLE))) {
> > > +        /* Triple fault is non-recoverable. */
> > > +        if (unlikely(tdx_check_exit_reason(vcpu, 
> > > EXIT_REASON_TRIPLE_FAULT)))
> > > +            return tdx_handle_triple_fault(vcpu);
> > > +
> > > +        kvm_pr_unimpl("TD vp_enter_ret 0x%llx, hkid 0x%x hkid pa 
> > > 0x%llx\n",
> > > +                  vp_enter_ret, to_kvm_tdx(vcpu->kvm)->hkid,
> > > +                  set_hkid_to_hpa(0, to_kvm_tdx(vcpu->kvm)->hkid));
> >
> > It indeed needs clarification for the need of "hkid" and "hkid pa". 
> > Especially the "hkdi pa", which is the result of applying HKID of the 
> > current TD to a physical address 0. I cannot think of any reason why 
> > we need such info.
> Yes, set_hkid_to_hpa(0, to_kvm_tdx(vcpu->kvm)->hkid) should be removed.
> I didn't notice it.

don't forget to justify why HKID is useful here. To me, HKID can be 
dropped as well.

> Thanks!
>
>
> > > +        goto unhandled_exit;
> > > +    }
> > > +
> > > +    /* From now, the seamcall status should be TDX_SUCCESS. */
> > > +    WARN_ON_ONCE((vp_enter_ret & TDX_SEAMCALL_STATUS_MASK) != 
> > > TDX_SUCCESS);
> >
> > Is there any case that TDX_SUCCESS with additional non-zero 
> > information in the lower 32-bits? I thought TDX_SUCCESS is a whole 64- 
> > bit status code.
> TDX status code uses the upper 32-bits.
>
> When the status code is TDX_SUCCESS and has a valid VMX exit reason, the 
> lower
> 32-bit is the VMX exit reason.
>
> You can refer to the TDX module ABI spec or 
> interface_function_completion_status.json
> from the intel-tdx-module-1.5-abi-table for details.

I see. (I asked a silly question that I even missed the normal Exit case)