A native virtio-blk boot ROM.
The gPXE network bootloader has virtio-net support and block device functionality used for iSCSI and ATA-over-Ethernet. It should be possible to add a virtio-blk driver, sharing much of the virtio-net infrastructure, and connecting it to the block device layer. The result will be a boot ROM suitable for virtio-blk PCI adapters.
As a result of this native virtio-blk boot ROM, the extboot ROM and its guest interface will not be needed for virtio-blk boot.
Laurent Vivier has done work in this area but the patches have not been merged into gPXE. I (Stefan Hajnoczi) have also written a virtio-blk driver for gPXE. I am going to unify these patches and propose them for merge into gPXE mainline.