[comboot] Run com32 programs with a valid IDT
COM32 binaries generally expect to run with interrupts enabled. Syslinux does
so, and COM32 programs will execute cli/sti pairs when running a critical
section, to provide mutual exclusion against BIOS interrupt handlers.
Previously, under gPXE, the IDT was not valid, so any interrupt (e.g. a timer
tick) would generally cause the machine to triple fault.
This change introduces code to:
- Create a valid IDT at the same location that syslinux uses
- Create an "interrupt jump buffer", which contains small pieces of code that
simply record the vector number and jump to a common handler
- Thunk down to real mode and execute the BIOS's interrupt handler whenever
an interrupt is received in a COM32 program
- Switch IDTs and enable/disable interrupts when context switching to and from
COM32 binaries
Testing done:
- Booted VMware ESX using a COM32 multiboot loader (mboot.c32)
- Built with GDBSERIAL enabled, and tested breakpoints on int22 and com32_irq
- Put the following code in a COM32 program:
asm volatile ( "sti" );
while ( 1 );
Before this change, the machine would triple fault immediately. After this
change, it hangs as expected. Under Bochs, it is possible to see the
interrupt handler run, and the current time in the BIOS data area gets
incremented.
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>