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

Binbin Wu
2024-12-10T02:14:40Z

On 12/9/2024 7:21 PM, Chao Gao wrote:
> On Sun, Dec 01, 2024 at 11:53:50AM +0800, Binbin Wu wrote:
>> From: Isaku Yamahata <isaku.yamahata@intel.com>
>>
>> Introduce the wiring for handling TDX VM exits by implementing the
>> callbacks .get_exit_info(), and .handle_exit(). Additionally, add
>> error handling during the TDX VM exit flow, and add a place holder
>> to handle various exit reasons. Add helper functions to retrieve
>> exit information, exit qualifications, and more.
>>
>> Contention Handling: The TDH.VP.ENTER operation may contend with TDH.MEM.*
>> operations for secure EPT or TD EPOCH. If contention occurs, the return
>> value will have TDX_OPERAND_BUSY set with operand type, prompting the vCPU
>> to attempt re-entry into the guest via the fast path.
>>
>> Error Handling: The following scenarios will return to userspace with
>> KVM_EXIT_INTERNAL_ERROR.
>> - TDX_SW_ERROR: This includes #UD caused by SEAMCALL instruction if the
>> CPU isn't in VMX operation, #GP caused by SEAMCALL instruction when TDX
>> isn't enabled by the BIOS, and TDX_SEAMCALL_VMFAILINVALID when SEAM
>> firmware is not loaded or disabled.
>> - TDX_ERROR: This indicates some check failed in the TDX module, preventing
>> the vCPU from running.
>> - TDX_NON_RECOVERABLE: Set by the TDX module when the error is
>> non-recoverable, indicating that the TDX guest is dead or the vCPU is
>> disabled. This also covers failed_vmentry case, which must have
>> TDX_NON_RECOVERABLE set since off-TD debug feature has not been enabled.
>> An exception is the triple fault, which also sets TDX_NON_RECOVERABLE
>> but exits to userspace with KVM_EXIT_SHUTDOWN, aligning with the VMX
>> case.
>> - Any unhandled VM exit reason will also return to userspace with
>> KVM_EXIT_INTERNAL_ERROR.
>>
>> Suggested-by: Sean Christopherson <seanjc@google.com>
>> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
>> Co-developed-by: Binbin Wu <binbin.wu@linux.intel.com>
>> Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
> Reviewed-by: Chao Gao <chao.gao@intel.com>
>
> [..]
>
>> fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
>> {
>> struct vcpu_tdx *tdx = to_tdx(vcpu);
>> @@ -837,9 +900,26 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
>> tdx->prep_switch_state = TDX_PREP_SW_STATE_UNRESTORED;
>>
>> vcpu->arch.regs_avail &= ~VMX_REGS_LAZY_LOAD_SET;
>> +
>> + if (unlikely((tdx->vp_enter_ret & TDX_SW_ERROR) == TDX_SW_ERROR))
>> + return EXIT_FASTPATH_NONE;
>> +
>> + if (unlikely(tdx_check_exit_reason(vcpu, EXIT_REASON_MCE_DURING_VMENTRY)))
>> + kvm_machine_check();
> I was wandering if EXIT_REASON_MCE_DURING_VMENTRY should be handled in the
> switch-case in tdx_handle_exit() because I saw there is a dedicated handler
> for VMX. But looks EXIT_REASON_MCE_DURING_VMENTRY is a kind of VMentry
> failure. So, it won't reach that switch-case.
Yes, similar to VMX, for TDX, EXIT_REASON_MCE_DURING_VMENTRY will be captured
by failed_vmentry, which will have TDX_NON_RECOVERABLE set and handled before
the switch-case.

> And, VMX's handler for
> EXIT_REASON_MCE_DURING_VMENTRY is actually dead code and can be removed.
I think so, for VMX, the exit_reason.failed_vmentry case will return to
userspace, it won't reach the handler for EXIT_REASON_MCE_DURING_VMENTRY.

>
>> +
>> trace_kvm_exit(vcpu, KVM_ISA_VMX);
>>
>> - return EXIT_FASTPATH_NONE;
>> + if (unlikely(tdx_has_exit_reason(vcpu) && tdexit_exit_reason(vcpu).failed_vmentry))
>> + return EXIT_FASTPATH_NONE;
>> +
>> + return tdx_exit_handlers_fastpath(vcpu);
>> +}
>> +
>> +static int tdx_handle_triple_fault(struct kvm_vcpu *vcpu)
>> +{
>> + vcpu->run->exit_reason = KVM_EXIT_SHUTDOWN;
>> + vcpu->mmio_needed = 0;
>> + return 0;
>> }
>>
>> 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.
>> + */
>> + 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));
>> + goto unhandled_exit;
>> + }
>> +
>> + /* From now, the seamcall status should be TDX_SUCCESS. */
>> + WARN_ON_ONCE((vp_enter_ret & TDX_SEAMCALL_STATUS_MASK) != TDX_SUCCESS);
>> + exit_reason = tdexit_exit_reason(vcpu);
>> +
>> + switch (exit_reason.basic) {
>> + default:
>> + break;
>> + }
>> +
>> +unhandled_exit:
>> + vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
>> + vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON;
>> + vcpu->run->internal.ndata = 2;
>> + vcpu->run->internal.data[0] = vp_enter_ret;
>> + vcpu->run->internal.data[1] = vcpu->arch.last_vmentry_cpu;
>> + return 0;
>> +}
>> +
>> +void tdx_get_exit_info(struct kvm_vcpu *vcpu, u32 *reason,
>> + u64 *info1, u64 *info2, u32 *intr_info, u32 *error_code)
>> +{
>> + struct vcpu_tdx *tdx = to_tdx(vcpu);
>> +
>> + if (tdx_has_exit_reason(vcpu)) {
>> + /*
>> + * Encode some useful info from the the 64 bit return code
>> + * into the 32 bit exit 'reason'. If the VMX exit reason is
>> + * valid, just set it to those bits.
>> + */
>> + *reason = (u32)tdx->vp_enter_ret;
>> + *info1 = tdexit_exit_qual(vcpu);
>> + *info2 = tdexit_ext_exit_qual(vcpu);
>> + } else {
>> + /*
>> + * When the VMX exit reason in vp_enter_ret is not valid,
>> + * overload the VMX_EXIT_REASONS_FAILED_VMENTRY bit (31) to
>> + * mean the vmexit code is not valid. Set the other bits to
>> + * try to avoid picking a value that may someday be a valid
>> + * VMX exit code.
>> + */
>> + *reason = 0xFFFFFFFF;
>> + *info1 = 0;
>> + *info2 = 0;
>> + }
>> +
>> + *intr_info = tdexit_intr_info(vcpu);
> If there is no valid exit reason, shouldn't intr_info be set to 0?
Yes. Will fix it.
Thanks!

>
>> + *error_code = 0;
>> +}
>> +