Hello Andrey,
The linked article, under the "Advanced Options" title (just past middle of page) covers two interesting aspects of non-tftp transfer alternatives.
One is "Faster Transfers With HTTP",
The other is NFS.
Neither method / description has anything to do with issue of mount_nfs after the kernel has been loaded.
> Implementing NFS client in grub is a lot of work for very little gain,
> especially if you are speaking about PXE which mandates working tftp
> server. What exact functionality is missing right now?
The advantage of using http or nfs compared to tftp is transfer speed, as summarized on the page:
"TFTP is not particularly fast when transferring the 200M or larger compressed filesystems used by some bootable utilities. Newer utilities can often retrieve their filesystem images by HTTP, which is much faster. One target system took three minutes to load the sysrcd.dat file by TFTP. With HTTP, it took only 18 seconds."
The article describes NFS as an alternative to HTTP, with conceivably similar file transfer speed.
Of course, the grub developers may have decided that "grub is a boot loader and not a PXE oriented project"; which is of course fine. As stated, I just wanted to get your view(s) on the described alternative boot method.
Regards.