[SeaBIOS] [Qemu-devel] [RFC] Passing boot order from qemu to seabios

H. Peter Anvin hpa at zytor.com
Mon Oct 11 23:51:40 CEST 2010

On 10/11/2010 02:41 PM, Sebastian Herbszt wrote:
> H. Peter Anvin wrote:
>> On 10/11/2010 01:30 PM, Anthony Liguori wrote:
>>> On 10/11/2010 02:59 PM, Gleb Natapov wrote:
>>>> No boot rom should do that. extboot wreaks havoc when it is used.
>>>> And since virtio is now supported by bios there is no reason to use it.
>>> You don't really have a choice.  You could be doing hardware passthrough 
>>> and the ROM on the card may hijack int19.
>> The BBS standard actually documents how to deal with that -- it pretty
>> much works out to "let the card initialize, then see if it mucked with
>> int19, and then put int19 back... if we want to run that card, then we
>> invoke the int19 that the card set up."
> The BIOS Boot Specification, Version 1.01 from January 11, 1996 seems not to
> recommend this:
> 3.4 Legacy IPL Devices
> "Legacy IPL devices will be allowed to take control of the system (via hooking
> interrupts) in both Legacy and PnP systems. The Plug and Play BIOS specification
> recommends that Legacy devices that hook a bootstrap interrupt such as INT 19h, 18h,
> or 13h have the interrupt re-captured by the BIOS. This is not done because grabbing
> an interrupt vector back after a device has hooked it can produce unpredictable results.
> Further, by allowing the card to take control, the behavior of these Legacy cards will
> be the same on both PnP and Legacy machines."
> 6.8 Notes on the POST Process
> "The Plug and Play BIOS Specification says that if a Legacy IPL device's option
> ROM captures INT 18h or INT 19h, the BIOS should save this vector and then
> restore the original one put there by the BIOS. The BIOS Boot Specification
> deviates from this in that these vectors are not recaptured after each Legacy option
> ROM returns from initialization. That would be considered unsafe."

Sorry, you're right -- I confused the PNPBIOS spec with the BBS spec
(and compounded the error by correctly remembering that BBS overrides


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