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. > > 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.
