Earlier this month, Laszlo Ersek of Redhat contributed a large patch to the TianoCore project, adding SMM support to OVMF.
“OVMF is capable of utilizing SMM if the underlying QEMU or KVM hypervisor emulates SMM. SMM is put to use in the S3 suspend and resume infrastructure, and in the UEFI variable driver stack. The purpose is (virtual) hardware separation between the runtime guest OS and the firmware (OVMF), with the intent to make Secure Boot actually secure, by preventing the runtime guest OS from tampering with the variable store and S3 areas.”
The changes require QEMU usage with these flags:
qemu-system-i386-machine q35,smm=on,accel=(tcg|kvm)-global driver=cfi.pflash01,property=secure,value=on -smp cpus=1 …
This new OVMF SMM works on the q35 machine, only in uniprocessor guests, and with TCG acceleration it only works on x86 not x64. Apparently the lack of 64-bit support is due to 64-bit code not available in TianoCore(?):
“In addition, using OvmfPkgIa32X64.dsc or OvmfPkgX64.dsc, the patch set even stops building after a point, *if* -D SMM_REQUIRE is passed. This is due to the unavailability of 64-bit open source components from Intel, and the build breakage is fully intentional — it shows that the -D SMM_REQUIRE feature is build-level incomplete for OvmfPkgIa32X64.dsc and OvmfPkgX64.dsc, and marks precisely where further development is needed.”
Check out the 58-part patch on the mailing list, the first and last messages have a lot more documentation: