PXE boot

[John Baldwin]

On Wednesday, September 03, 2014 11:16:05 PM Sinha, Prokash wrote:
> Thanks much, John !
> 
> When it starts executing ? How ?. Who starts executing these. From the
> Makefile it's org is 0x7c00. So I would assume that it would be loaded at
> that absolute real-mode address !. I don't understand this part. But, if I
> put traces, I see pxe_open, and pxe_close are being called, but after all
> these pxe_calls are done, the loader's main is invoked at Rebooting Š time.
> 
> We are using a net boot server, and I see mountd- authenticated Š message
> storm on the boot server side.
> 
> Wondering what could cause such a message storm, while pxe_open() -> net
> if_open( ) executes.

The PXE BIOS uses TFTP to fetch the pxeboot binary (and it will do its own 
DHCP, etc. as part of doing that).  It writes the binary it downloads starting 
at address 0x7c00.  Once the download is complete, it jumps to 0x7c00 similar 
to how booting from a disk loads the first sector at address 0x7c00 and then 
jumps to it.

The PXE calls in libi386 should not be invoked until the loader main() routine 
runs:

    /*
     * Special handling for PXE and CD booting.
     */
    if (kargs->bootinfo == 0) {
	/*
	 * We only want the PXE disk to try to init itself in the below
	 * walk through devsw if we actually booted off of PXE.
	 */
	if (kargs->bootflags & KARGS_FLAGS_PXE)
	    pxe_enable(kargs->pxeinfo ? PTOV(kargs->pxeinfo) : NULL);
	else if (kargs->bootflags & KARGS_FLAGS_CD)
	    bc_add(initial_bootdev);
    }

That enables the PXE devsw driver so that it will do something later when 
main() calls all the devsw init routines:

    /*
     * March through the device switch probing for things.
     */
    for (i = 0; devsw[i] != NULL; i++)
	if (devsw[i]->dv_init != NULL)
	    (devsw[i]->dv_init)();
    printf("BIOS %dkB/%dkB available memory\n", bios_basemem / 1024, 
bios_extmem / 1024);

-- 
John Baldwin