[gPXE] [Etherboot-discuss] Boot from one card only

[Stefan Hajnoczi]

On Tue, Jan 12, 2010 at 9:54 AM, Itay Gazit <itaygazit at gmail.com> wrote:
> In order to implement what you have suggested, I will need to use NVRAM to
> store the PCI device function, right?

No, you do not need NVRAM.

When the BIOS calls the gPXE ROM entry point it passes the PCI
bus:dev.fn in a register.  Right now this register is not stored away
anywhere so gPXE "forgets" which device it came from.

Storing away the PCI bus:dev.fn is a little bit tricky because the
gPXE image is split into .prefix, .text16/.data16, and .text/.data
sections.

.prefix is the uncompressed startup code which is called by the BIOS.
It uncompresses gPXE into memory and passes control to it.  Once gPXE
proper has started up the .prefix section is abandoned and may be
overwritten.

.text16/.data16 is for 16-bit code and lives <1 MB in the address
space.  This section is stored compressed in the gPXE image.

.text/.data is for 32-bit code and is loaded at a suitable location >1
MB in the address space.  This section is stored compressed in the
gPXE image.

The .prefix code mostly runs in 16-bit real-mode and is not able to
directly access .text/.data.  Therefore variables that are accessed by
both 16-bit real-mode and 32-bit protected mode code usually live in
the .data16 section.

The goal here is pass a value from .prefix code all the way through to
.text32 code.  This can be done by copying the value into a .data16
variable from .prefix code before entering gPXE.  The .text code may
access .data16 variables.

As an example look at arch/i386/prefix/pxeprefix.S:308 where the PXE
prefix stores away the PCI bus:dev.fn in the .prefix section.  Later,
on line 702, the UNDI device structure with the PCI bus:dev.fn is
copied to its final destination in the .text16 section.

The C code (32-bit .text code) declares the UNDI device structure as
follows in arch/i386/drivers/net/undipreload.c:
struct undi_device __data16 ( preloaded_undi );

Another thing to watch out for is the source tree structure where all
architecture-specific code goes into arch/{i386,x86,x86_64}.  The
main, portable, part of gPXE in usr/autoboot.c needs to get at the
network device in a way is not specific to the .rom prefix or the i386
architecture.

Hope this helps,
Stefan