Re: [PATCH 1/2] KVM: TDX: Enable Notify VM exit

[Nikolay Borisov <nik.borisov@suse.com>]

On 8/5/26 06:12, Xiaoyao Li wrote:
> Enable Notify VM exit functionality for TDX guests.
> 
> Notify VM exit is an existing feature supported by KVM. Userspace can
> enable Notify VM exit through KVM_CAP_X86_NOTIFY_VMEXIT when it's
> reported as supported. However, KVM reports the support of this CAP just
> based on the hardware capability but doesn't differentiate between VMX
> and TDX. This leads to the issue that userspace can enable this cap for
> TDX guests without getting an error, but the feature is not actually
> enabled because KVM doesn't call the TDX module API to program the
> relevant TD VMCS fields.
> 
> Enable Notify VM exit for TDX guests by:
> 
>    - Invoking TDX module API calls to set NOTIFY_VM_EXITING and Notify
>      Window in TD VMCS. It's done in tdx_vcpu_init() where other TD VMCS
>      bits are set. Since TDX vCPU cannot be reset, it only needs to be
>      configured once when initializing the TDX vCPU.
> 
>    - Adding corresponding exit handler for TDX Notify VM Exit.


nit: That feature is completely misnamed in the kernel. It should be 
instruction timeout (as is in the SDM). Please reword the changelog to 
refer to the name of the features as they are in the SDM. I.e if you 
search for NOTIFY_VMEXIT or NOTIFY_WINDOW absolutely nothing can be 
found in the SDM. The changelog should ideally mention both - SDM's 
nomenclature and linux's nomenclature.
> 
> Note, Notify VM exit can happen when executing the IRET instruction. If
> the IRET unblocks the NMI blocking state, bit 12 of the exit qualification
> is set. In this case, the VMM needs to restore the "blocked by NMI" state
> when it decides to re-enter the guest. For TDX, KVM cannot manage the
> GUEST_INTERRUPTIBILITY_INFO and it's TDX module's responsibility to
> handle it.
> 
> Fixes: 161d34609f9b ("KVM: TDX: Make TDX VM type supported")
> Cc: stable@vger.kernel.org
> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> ---
> The enabling of Notify VM exit was missed in the initial upstreaming of
> TDX base support. We suppose the patch needs to be backported to
> stable kernels. So, the cc stable is added.
> ---
>   arch/x86/kvm/vmx/tdx.c | 10 ++++++++++
>   arch/x86/kvm/vmx/vmx.c | 23 +++++++++++++++--------
>   arch/x86/kvm/vmx/vmx.h |  1 +
>   3 files changed, 26 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 545b03d9d10b..cdc0d24657ac 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -2129,6 +2129,9 @@ int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
>   		 * - If it's not an MSMI, no need to do anything here.
>   		 */
>   		return 1;
> +	case EXIT_REASON_NOTIFY:
> +		/* NMI blocking state is handled by TDX module */
> +		return __handle_notify(vcpu, false);

I'd rather there be a private handle_tdx_notify function in tdx.c than 
exposing __handle_notify and introducing the boolean. This is needed 
because the TDX module handles the NMI unblocking, so let's keep the 
implementation specific to tdx.

<snip>