On 10/19/2023 12:50 AM, Sean Christopherson wrote:
> When vNMI is enabled, rely entirely on hardware to correctly handle NMI
> blocking, i.e. don't intercept IRET to detect when NMIs are no longer
> blocked. KVM already correctly ignores svm->nmi_masked when vNMI is
> enabled, so the effect of the bug is essentially an unnecessary VM-Exit.
>
> KVM intercepts IRET for two reasons:
> - To track NMI masking to be able to know at any point of time if NMI
> is masked.
> - To track NMI windows (to inject another NMI after the guest executes
> IRET, i.e. unblocks NMIs)
>
> When vNMI is enabled, both cases are handled by hardware:
> - NMI masking state resides in int_ctl.V_NMI_BLOCKING and can be read by
> KVM at will.
> - Hardware automatically "injects" pending virtual NMIs when virtual NMIs
> become unblocked.
>
> However, even though pending a virtual NMI for hardware to handle is the
> most common way to synthesize a guest NMI, KVM may still directly inject
> an NMI via when KVM is handling two "simultaneous" NMIs (see comments in
> process_nmi() for details on KVM's simultaneous NMI handling). Per AMD's
> APM, hardware sets the BLOCKING flag when software directly injects an NMI
> as well, i.e. KVM doesn't need to manually mark vNMIs as blocked:
>
> If Event Injection is used to inject an NMI when NMI Virtualization is
> enabled, VMRUN sets V_NMI_MASK in the guest state.
>
> Note, it's still possible that KVM could trigger a spurious IRET VM-Exit.
> When running a nested guest, KVM disables vNMI for L2 and thus will enable
> IRET interception (in both vmcb01 and vmcb02) while running L2 reason. If
> a nested VM-Exit happens before L2 executes IRET, KVM can end up running
> L1 with vNMI enable and IRET intercepted. This is also a benign bug, and
> even less likely to happen, i.e. can be safely punted to a future fix.
>
> Fixes: fa4c027a7956 ("KVM: x86: Add support for SVM's Virtual NMI")
> Link: https://lore.kernel.org/all/ZOdnuDZUd4mevCqe@xxxxxxxxxxx
> Cc: Santosh Shukla <santosh.shukla@xxxxxxx>
> Cc: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>
> v2: Expand changelog to explain the various behaviors and combos. [Maxim]
>
> v1: https://lore.kernel.org/all/20231009212919.221810-1-seanjc@xxxxxxxxxx
>
Tested-by: Santosh Shukla <santosh.shukla@xxxxxxx>
Thanks,
