Block Driver and Virtio Blk are mentioned in the “libsel4vmmplatsupport” chapter, but they are not supported yet
original:
Potential future features (yet to be implemented)
Additional Virtio Driver Support
e.g Virtio Blk, Virtio RNG, Virtio Balloon
Block Driver support
To answer “How to access eMMC/ufs storage media in Guest VM” this is already possible via device passthrough. If you configure the interrupts and device registers to be made available to the guest, then the guest kernel is able to initialize a driver and access the hardware.
Additional Virtio drivers that aren’t already provided by libvmmplatsupport can still be written by a 3rd party, but there isn’t any ongoing work to provide them in libvmmplatsupport right now.