Hello Neil,
Thank you very much - at last I have a success!
You're also testing with the initramfs.cpio not the compressed initramfs.cpio.gz
That was really the issue. I didn't notice that LAVA put down into iPXE exactly `tftp .../ initramfs.cpio.gz`.
But I have to notice that I really need "extra_kernel_args: initrd=/ramdisk.cpio.gz root=/dev/ram0".
I attach my job definition and full log. Image to load have been generated on my Debian machine with script https://git.linaro.org/lava-team/refactoring.git/tree/scripts/x86_64-nfs.sh.
So I very appreciate you help and comments from Steve McIntyre.
With best regards Sergey Kovalev.