Re: [PATCH 0/7] KVM: TDX: TD vcpu enter/exit

[Binbin Wu <binbin.wu@linux.intel.com>]

On 11/26/2024 11:52 AM, Huang, Kai wrote:
> On Tue, 2024-11-26 at 09:44 +0800, Binbin Wu wrote:
>>
>> On 11/26/2024 6:51 AM, Sean Christopherson wrote:
>>
>> [...]
>>> When an NMI happens in non-root, the NMI is acknowledged by the CPU prior to
>>> performing VM-Exit.  In regular VMX, NMIs are blocked after such VM-Exits.  With
>>> TDX, that blocking happens for SEAM root, but the SEAMRET back to VMX root will
>>> load interruptibility from the SEAMCALL VMCS, and I don't see any code in the
>>> TDX-Module that propagates that blocking to SEAMCALL VMCS.
>> I see, thanks for the explanation!
>>
>>> Hmm, actually, this means that TDX has a causality inversion, which may become
>>> visible with FRED's NMI source reporting.  E.g. NMI X arrives in SEAM non-root
>>> and triggers a VM-Exit.  NMI X+1 becomes pending while SEAM root is active.
>>> TDX-Module SEAMRETs to VMX root, NMIs are unblocked, and so NMI X+1 is delivered
>>> and handled before NMI X.
>> This example can also cause an issue without FRED.
>> 1. NMI X arrives in SEAM non-root and triggers a VM-Exit.
>> 2. NMI X+1 becomes pending while SEAM root is active.
>> 3. TDX-Module SEAMRETs to VMX root, NMIs are unblocked.
>> 4. NMI X+1 is delivered and handled before NMI X.
>>      (NMI handler could handle all NMI source events, including the source
>>       triggered NMI X)
>> 5. KVM calls exc_nmi() to handle the VM Exit caused by NMI X
>> In step 5, because the source event caused NMI X has been handled, and NMI X
>> will not be detected as a second half of back-to-back NMIs, according to
>> Linux NMI handler, it will be considered as an unknown NMI.
> I don't think KVM should call exc_nmi() anymore if NMI is unblocked upon
> SEAMRET.

IIUC, KVM has to, because the NMI triggered the VM-Exit can't trigger the
NMI handler to be invoked automatically even if NMI is unblocked upon SEAMRET.

>
>> Actually, the issue could happen if NMI X+1 occurs after exiting to SEAM root
>> mode and before KVM handling the VM-Exit caused by NMI X.
>>
> If we can make sure NMI is still blocked upon SEAMRET then everything follows
> the current VMX flow IIUC.  We should make that happen IMHO.
>
>
Agree.