Re: [Qemu-devel] [PATCH 1/2] pc: make vgabios exit port more useful

[Anthony Liguori]

On 08/24/2011 04:47 AM, Avi Kivity wrote:
> On 08/08/2011 10:31 PM, Anthony Liguori wrote:
> > We've always listened on port 501 for vgabios panic messages. In the
> > entire
> > time I've worked on QEMU, I've never actually seen a vgabios panic
> > message :-)
> >
> > If we change the semantics of this port a little bit, it makes it
> > possible to
> > use it for more interesting use-cases. I chose this approach instead
> > of adding
> > a new I/O port because it avoids having a guest visible change.
> >
> > This change allows single-byte access to port 501 and also uses the value
> > written to construct an exit code.
>
> A little late to review but...
>
> > diff --git a/hw/pc.c b/hw/pc.c
> > index 1c9d89a..4b07b35 100644
> > --- a/hw/pc.c
> > +++ b/hw/pc.c
> > @@ -549,8 +549,7 @@ static void bochs_bios_write(void *opaque,
> > uint32_t addr, uint32_t val)
> > /* LGPL'ed VGA BIOS messages */
> > case 0x501:
> > case 0x502:
> > - fprintf(stderr, "VGA BIOS panic, line %d\n", val);
> > - exit(1);
> > + exit((val<< 1) | 1);
>
> This code (before the patch) circumvents -no-shutdown.

Indeed.  I believe that's a feature though?

> Shifting val left is surprising. What's wrong with even exit codes?

Hrm, the '| 1' is wrong.  My intention was to make the bottom bit of the 
exit code mean "if set to 0, this is an exit coming from a guest'.

I'll spin a patch.

Regards,

Anthony Liguori

>