[SeaBIOS] Problem with Debug lvl under XEN

Kevin O'Connor kevin at koconnor.net
Tue Feb 14 01:33:43 CET 2012

On Mon, Feb 13, 2012 at 08:50:56PM +0000, Ian Campbell wrote:
> On Mon, 2012-02-13 at 23:21 +0900, Daniel Castro wrote:
> > Hello,
> > 
> > I have encountered something a little strange, if I set up the debug
> > lvl to 3 or more Y will get a Triple VCPU fault. If I set it to 1 the
> > bios runs normally but I loose a lot of information that I need to
> > debug. Sometimes if I try to print char * variables regardless of the
> > debug level I still get the fault.
> > 
> > Any ideas why?
> My guess is that there is a debug print at lvl>=3 which ends up
> dereferencing a NULL pointer in one of its arguments (probably a %s) and
> this leads to a page fault. This in turn leads to a double fault because
> SeaBIOS does not install a page fault handler and then a triple fault
> because it also does not install a double fault handler. Likewise when
> you are printing "char * variables regardless of the debug level".

SeaBIOS doesn't have paging enabled, so it should not need to install
a page fault handler.  SeaBIOS needs to write the real-mode interrupt
descriptor table to address 0, so it should definitely have read/write
access to the memory there.  Thus, a null pointer dereference
shouldn't cause a fault.  Indeed, I can't think of much that should
cause a fault (other than read/write to IO memory incorrectly, divide
by zero, invalid opcode, etc.).

> You could test this by adding an explicit check for null in the bit of
> bvprintf which handles %s, perhaps putc()ing "(null)" instead.

If you think it is specific to the Xen handling, one could also try
running the same code on qemu to verify it.


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