Re: Hook INT18 for IBM servers?

Does anyone know, how to save the INT18 vector and restore it back after
gPXE executed?

On Wed, Aug 26, 2009 at 2:04 AM, Jarrod Johnson
<[email protected]>wrote:

> IBM BIOS can rehook INT19 to prevent gPXE from jumping in front, but
> without hooking anything, I don't see a way to make gPXE actually boot
> (i.e. the BEV never gets called by IBM BIOS).  If I make gPXE hook
> INT18, then it certainly does proceed in the expected order, but I
> can't get past the gPXE boot device without looping to the first
> network interface (if storing and calling the original INT18 vector)
> or hanging (various attempts to ret/iret out of the INT18 handler).
> Glenn Brown at Myricom managed to get Etherboot to work like this, but
> I can't figure it out for gPXE (maybe I'm not ensuring the stack is
> restored or some register value is right when I iret, or maybe I just
> am out of my depth generally).
>
> My understanding of how IBM BIOS works:
> -It executes the PCI option rom
> -If option rom hooks int18, it saves that int18 vector and reinstates its
> own
> -When 'network boot' option is reached, it invokes its INT18 handler
> -That handler then, in order, invokes each INT18 handler
> -If an INT18 handler wants to yield to the next device, it achieves it
> in IBM BIOS by iret to get back in the correct place to IBM INT18
> handler.  Presumably, the stack and possibly other registers are
> expected to be put back the way they were when the option rom started
> execution.
>
> I'm a little outside my general scope (firmware development is
> something I personally lack experience in), but I'll do what I can to
> help as possible.
>
> On Tue, Aug 25, 2009 at 1:23 PM, Michael Brown<[email protected]>
> wrote:
> > On Monday 24 August 2009 19:13:43 Jarrod Johnson wrote:
> >> In short, the GIT change did allow gPXE to detect PnP capability, but
> >> INT19 is still detected/used, and I'm pretty sure that is an accurate
> >> assessment of IBM BIOS capabilities (PnP, but not BBS compliant).
> >
> > Thanks for testing this.  I was waiting for an opportunity to get hold of
> an
> > IBM to test against.
> >
> > I believe that most IBM BIOSes usually have some form of "recapture INT
> 19"
> > option.  If you use this, you should find that the BIOS overwrites the
> INT 19
> > value written by gPXE, which will prevent gPXE interfering with the boot
> > process.  You *should* then be able to select gPXE via whatever
> > non-BBS-compliant boot device selection mechanism the BIOS provides.
> >
> > This is based only partly on memory; I may be wrong on some of the
> details.
> >
> > Michael
> >
>

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