Date: Thu, 04 Sep 2014 10:57:56 -0400 From: John Baldwin <jhb@freebsd.org> To: "Sinha, Prokash" <psinha@panasas.com> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org> Subject: Re: PXE boot Message-ID: <8193889.CqHK0JhjBu@ralph.baldwin.cx> In-Reply-To: <D02CE8DA.17D2%psinha@panasas.com> References: <D02C8BE1.179F%psinha@panasas.com> <1641492.zXWdUoX4Sh@ralph.baldwin.cx> <D02CE8DA.17D2%psinha@panasas.com>
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On Wednesday, September 03, 2014 11:16:05 PM Sinha, Prokash wrote:
> Thanks much, John !
>=20
> When it starts executing ? How ?. Who starts executing these. From th=
e
> Makefile it's org is 0x7c00. So I would assume that it would be loade=
d 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 =C5=
=A0 time.
>=20
> We are using a net boot server, and I see mountd- authenticated =C5=A0=
message
> storm on the boot server side.
>=20
> 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=20
DHCP, etc. as part of doing that). It writes the binary it downloads s=
tarting=20
at address 0x7c00. Once the download is complete, it jumps to 0x7c00 s=
imilar=20
to how booting from a disk loads the first sector at address 0x7c00 and=
then=20
jumps to it.
The PXE calls in libi386 should not be invoked until the loader main() =
routine=20
runs:
/*
* Special handling for PXE and CD booting.
*/
if (kargs->bootinfo =3D=3D 0) {
=09/*
=09 * We only want the PXE disk to try to init itself in the below
=09 * walk through devsw if we actually booted off of PXE.
=09 */
=09if (kargs->bootflags & KARGS_FLAGS_PXE)
=09 pxe_enable(kargs->pxeinfo ? PTOV(kargs->pxeinfo) : NULL);
=09else if (kargs->bootflags & KARGS_FLAGS_CD)
=09 bc_add(initial_bootdev);
}
That enables the PXE devsw driver so that it will do something later wh=
en=20
main() calls all the devsw init routines:
/*
* March through the device switch probing for things.
*/
for (i =3D 0; devsw[i] !=3D NULL; i++)
=09if (devsw[i]->dv_init !=3D NULL)
=09 (devsw[i]->dv_init)();
printf("BIOS %dkB/%dkB available memory\n", bios_basemem / 1024,=20=
bios_extmem / 1024);
--=20
John Baldwin
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